* Wrap `auth.get_user_by_req` in an opentracing span
give `get_user_by_req` its own opentracing span, since it can result in a
non-trivial number of sub-spans which it is useful to group together.
This requires a bit of reorganisation because it also sets some tags (and may
force tracing) on the servlet span.
* Emit opentracing span for encoding json responses
This can be a significant time sink.
* Rename all sync spans with a prefix
* Write an opentracing span for encoding sync response
* opentracing span to group generate_room_entries
* opentracing spans within sync.encode_response
* changelog
* Use the `trace` decorator instead of context managers
