Support filtering refUpdate events for NoteDb meta refs

Ref update events on notedb meta refs are not useful and likely won't
ever be consumed by external users as it is an internal Gerrit ref. Add
support for a DROP rule to filter them out:

 DROP RefUpdatedEvent isNoteDbMetaRef

On our busy servers with too many writes on changes it seems to take too
long to store events, this sometimes causes other writes to suffer so
hopefully filtering these out will help.

Release-Notes: NoteDb refUpdatedEvents can now be filtered
Change-Id: I813d085d8a0ea5c6cdeb83753f97e8ee50727370
3 files changed