CONTRIB: opentracing: add the OpenTracing filter

This commit adds the OpenTracing filter (hereinafter we will use the
abbreviated name 'the OT filter') to the contrib tree.

The OT filter adds native support for using distributed tracing in HAProxy.
This is enabled by sending an OpenTracing compliant request to one of the
supported tracers; such as Datadog, Jaeger, Lightstep and Zipkin tracers.
Please note: tracers are not listed by any preference, but alphabetically.

The OT filter is a standard HAProxy filter, so what applies to others also
applies to this one (of course, by that I mean what is described in the
documentation, more precisely in the doc/internals/filters.txt file).

The OT filter activation is done explicitly by specifying it in the HAProxy
configuration.  If this is not done, the OT filter in no way participates
in the work of HAProxy.

As for the impact on HAProxy speed, this is documented with several tests
located in the test directory, and the result is found in the README-speed-*
files.  In short, the speed of operation depends on the way it is used and
the complexity of the configuration, from an almost immeasurable impact to
a significant deceleration (5x and more).  I think that in some normal use
the speed of HAProxy with the filter on will be quite satisfactory with a
slowdown of less than 4%.

The OT filter allows intensive use of ACLs, which can be defined anywhere in
the configuration.  Thus, it is possible to use the filter only for those
connections that are of interest to us.

More detailed documentation related to the operation, configuration and use
of the filter can be found in the contrib/opentracing directory.

To make the OpenTracing filter easier to configure and compile, several
entries have been added to the Makefile.  When running the make utility,
it is possible to use several new arguments:

  USE_OT=1     : enable the OpenTracing filter
  OT_DEBUG=1   : compile the OpenTracing filter in debug mode
  OT_INC=path  : force the include path to libopentracing-c-wrapper
  OT_LIB=path  : force the lib path to libopentracing-c-wrapper
  OT_RUNPATH=1 : add libopentracing-c-wrapper RUNPATH to haproxy executable

If USE_OT is set, then an additional Makefile from the contrib/opentracing
directory is included in the compilation process.
diff --git a/contrib/opentracing/test/README-speed-fe-be b/contrib/opentracing/test/README-speed-fe-be
new file mode 100644
index 0000000..ab2b7af
--- /dev/null
+++ b/contrib/opentracing/test/README-speed-fe-be
@@ -0,0 +1,111 @@
+--- rate-limit 100.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency     0.89ms  466.84us  35.44ms   94.39%
+    Req/Sec     1.09k    39.30     1.32k    72.60%
+  Latency Distribution
+     50%  823.00us
+     75%    1.00ms
+     90%    1.20ms
+     99%    2.14ms
+  2594524 requests in 5.00m, 687.86MB read
+Requests/sec:   8645.83
+Transfer/sec:      2.29MB
+----------------------------------------------------------------------
+
+--- rate-limit 50.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   681.74us  463.28us  20.45ms   95.46%
+    Req/Sec     1.41k    54.00     1.60k    68.97%
+  Latency Distribution
+     50%  613.00us
+     75%  785.00us
+     90%    0.98ms
+     99%    2.06ms
+  3367473 requests in 5.00m, 0.87GB read
+Requests/sec:  11222.76
+Transfer/sec:      2.98MB
+----------------------------------------------------------------------
+
+--- rate-limit 10.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   558.32us  458.54us  29.40ms   97.73%
+    Req/Sec     1.72k    60.67     2.05k    73.10%
+  Latency Distribution
+     50%  494.00us
+     75%  610.00us
+     90%  743.00us
+     99%    2.08ms
+  4105420 requests in 5.00m, 1.06GB read
+Requests/sec:  13683.36
+Transfer/sec:      3.63MB
+----------------------------------------------------------------------
+
+--- rate-limit 2.5 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   542.66us  440.31us  22.63ms   97.88%
+    Req/Sec     1.76k    60.02     2.00k    72.27%
+  Latency Distribution
+     50%  481.00us
+     75%  588.00us
+     90%  710.00us
+     99%    2.05ms
+  4214525 requests in 5.00m, 1.09GB read
+Requests/sec:  14046.76
+Transfer/sec:      3.72MB
+----------------------------------------------------------------------
+
+--- rate-limit 0.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   529.06us  414.38us  30.09ms   97.97%
+    Req/Sec     1.80k    59.34     2.05k    74.47%
+  Latency Distribution
+     50%  473.00us
+     75%  576.00us
+     90%  692.00us
+     99%    1.79ms
+  4287428 requests in 5.00m, 1.11GB read
+Requests/sec:  14290.45
+Transfer/sec:      3.79MB
+----------------------------------------------------------------------
+
+--- rate-limit disabled --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   517.81us  463.10us  36.81ms   98.25%
+    Req/Sec     1.85k    62.39     2.21k    75.65%
+  Latency Distribution
+     50%  458.00us
+     75%  558.00us
+     90%  670.00us
+     99%    1.96ms
+  4416273 requests in 5.00m, 1.14GB read
+Requests/sec:  14719.43
+Transfer/sec:      3.90MB
+----------------------------------------------------------------------
+
+--- rate-limit off --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   511.67us  428.18us  27.68ms   98.15%
+    Req/Sec     1.86k    60.67     2.05k    75.44%
+  Latency Distribution
+     50%  455.00us
+     75%  554.00us
+     90%  666.00us
+     99%    1.81ms
+  4441271 requests in 5.00m, 1.15GB read
+Requests/sec:  14803.32
+Transfer/sec:      3.92MB
+----------------------------------------------------------------------