BUG/MEDIUM: opentracing: initialization before establishing daemon and/or chroot mode

This patch solves the problem reported in github issue #1204, where the
OpenTracing filter cannot communicate with the selected tracer if HAProxy
is run in daemon mode.

This commit also solves github issue #1274, where the problem manifests
itself when using the 'chroot' keyword in the HAProxy configuration.

This is solved so that the initialization of the OpenTracing plugin is
split into two operations, first the plugin (dynamic library) is loaded
before switching the HAProxy to daemon mode (or chroot) and then the
tracer thread is started.

This means that nothing is retrieved from the file system in runtime.

After applying this commit, opentracing C wrapper version 1.1.0 should be
used because the earlier version does not have separated initialization
functions.

This resolves GitHub issues #1204 and #1274.

(cherry picked from commit 9425ed488f4d31db07f0c9b1e8b2f0fb65b18c7b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/addons/ot/include/opentracing.h b/addons/ot/include/opentracing.h
index 2dbf532..2b88a33 100644
--- a/addons/ot/include/opentracing.h
+++ b/addons/ot/include/opentracing.h
@@ -51,7 +51,8 @@
 void                     ot_text_map_show(const struct otc_text_map *text_map);
 void                     ot_debug(void);
 #endif
-int                      ot_init(struct otc_tracer **tracer, const char *config, const char *plugin, char **err);
+int                      ot_init(struct otc_tracer **tracer, const char *plugin, char **err);
+int                      ot_start(struct otc_tracer *tracer, const char *cfgbuf, char **err);
 struct otc_span         *ot_span_init(struct otc_tracer *tracer, const char *operation_name, const struct timespec *ts_steady, const struct timespec *ts_system, int ref_type, int ref_ctx_idx, const struct otc_span *ref_span, const struct otc_tag *tags, int num_tags, char **err);
 int                      ot_span_tag(struct otc_span *span, const struct otc_tag *tags, int num_tags);
 int                      ot_span_log(struct otc_span *span, const struct otc_log_field *log_fields, int num_fields);