Rename incr/decr to counter

This is a breaking change. Docs still need to be updated accordingly

Change-Id: Iedd6733ff77c0bbf05a9c363cd0e53a0004dc5b6
Reviewed-on: http://review.couchbase.org/41310
Reviewed-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>