CLEANUP: opentracing: use the haproxy function to generate uuid

CLEANUP: opentracing: use the haproxy function to generate uuid

To avoid duplicate source code, the original haproxy function is used to
generate the OpenTracing runtime context UUID.

Also, the structure flt_ot_runtime_context is simplified because the
detailed definition of UUID is removed from it (struct flt_ot_uuid),
ie the UUID is left only in the form of a string.