statsd: Rename MediawikiStatsdDataFactory to IBufferingStatsdDataFactory

Follows-up I2874175647e (7fdc3d09a3b).

* Avoid "MediaWiki" in class names. If unavoidable, always camelcase.
* Use a more descriptive name, matching the implementing the class ("Buffering").
* Use "I" prefix for consistency with other MediaWiki-provided interfaces.
  (Avoid "Interface" suffix, which is foreign to MediaWiki and is only used
  in the StatsdClient namespace, which this interface is not in).

Bug: T166354
Change-Id: I06de59122625f9c23e7c1a1bfa69a7ddabbf379e