Opentracing across workers (#5771)

Propagate opentracing contexts across workers


Also includes some Convenience modifications to opentracing for servlets, notably:
- Add boolean to skip the whitelisting check on inject
  extract methods. - useful when injecting into carriers
  locally. Otherwise we'd always have to include our
  own servername and whitelist our servername
- start_active_span_from_request instead of header
- Add boolean to decide whether to extract context
  from a request to a servlet

synapse/http/servlet.py

@@ -300,7 +300,7 @@ class RestServlet(object):
                     http_server.register_paths(
                         method,
                         patterns,
-                        trace_servlet(servlet_classname, method_handler),
+                        trace_servlet(servlet_classname)(method_handler),
                         servlet_classname,
                     )