Splitting this out from T288702: Reduce complexity and time spent in WANObjectCache.
Specifically, to eliminate the wall-time spent in Statsd-related code from the backend response times.
Note that for the past 5 years, it has already been the case that stats messages are buffered in-proces and sent to the Statsd server at the end of the request.
- Move any processing, filtering and formatting logic from individual timing() and increment()calls to the send phase.
- Confirm the send phase happens in a post-send step.