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-cmp b/contrib/opentracing/test/README-speed-cmp
new file mode 100644
index 0000000..9251faa
--- /dev/null
+++ b/contrib/opentracing/test/README-speed-cmp
@@ -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   650.95us  431.15us  46.44ms   96.67%
+    Req/Sec     1.44k    51.39     2.57k    74.89%
+  Latency Distribution
+     50%  608.00us
+     75%  760.00us
+     90%    0.91ms
+     99%    1.31ms
+  3434836 requests in 5.00m, 0.89GB read
+Requests/sec:  11446.99
+Transfer/sec:      3.03MB
+----------------------------------------------------------------------
+
+--- rate-limit 50.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   398.00us  371.39us  22.56ms   97.23%
+    Req/Sec     2.32k    84.01     2.76k    74.84%
+  Latency Distribution
+     50%  350.00us
+     75%  467.00us
+     90%  593.00us
+     99%    1.03ms
+  5530848 requests in 5.00m, 1.43GB read
+Requests/sec:  18434.31
+Transfer/sec:      4.89MB
+----------------------------------------------------------------------
+
+--- rate-limit 10.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   316.75us  351.92us  23.00ms   98.57%
+    Req/Sec     2.87k    94.02     3.22k    79.30%
+  Latency Distribution
+     50%  273.00us
+     75%  342.00us
+     90%  424.00us
+     99%    0.94ms
+  6859293 requests in 5.00m, 1.78GB read
+Requests/sec:  22862.16
+Transfer/sec:      6.06MB
+----------------------------------------------------------------------
+
+--- rate-limit 2.5 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   307.90us  368.64us  26.08ms   98.71%
+    Req/Sec     2.96k   103.84     3.23k    83.76%
+  Latency Distribution
+     50%  264.00us
+     75%  327.00us
+     90%  402.00us
+     99%    0.97ms
+  7065667 requests in 5.00m, 1.83GB read
+Requests/sec:  23550.37
+Transfer/sec:      6.24MB
+----------------------------------------------------------------------
+
+--- rate-limit 0.0 --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   304.60us  376.36us  30.26ms   98.74%
+    Req/Sec     2.99k   106.93     3.24k    83.08%
+  Latency Distribution
+     50%  262.00us
+     75%  323.00us
+     90%  396.00us
+     99%    0.95ms
+  7136261 requests in 5.00m, 1.85GB read
+Requests/sec:  23785.77
+Transfer/sec:      6.31MB
+----------------------------------------------------------------------
+
+--- rate-limit disabled --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   300.90us  342.35us  22.13ms   98.74%
+    Req/Sec     3.00k    95.67     3.33k    81.11%
+  Latency Distribution
+     50%  261.00us
+     75%  322.00us
+     90%  394.00us
+     99%  806.00us
+  7159525 requests in 5.00m, 1.85GB read
+Requests/sec:  23863.05
+Transfer/sec:      6.33MB
+----------------------------------------------------------------------
+
+--- rate-limit off --------------------------------------------------
+Running 5m test @ http://localhost:10080/index.html
+  8 threads and 8 connections
+  Thread Stats   Avg      Stdev     Max   +/- Stdev
+    Latency   302.51us  371.99us  30.26ms   98.77%
+    Req/Sec     3.00k   104.43     3.73k    83.74%
+  Latency Distribution
+     50%  260.00us
+     75%  321.00us
+     90%  394.00us
+     99%    0.89ms
+  7170345 requests in 5.00m, 1.86GB read
+Requests/sec:  23898.19
+Transfer/sec:      6.34MB
+----------------------------------------------------------------------