CCBC-486: Implement mcflush

This restores the old memcached flush command under the alias 'mcflush'.

This also fixes a bug with the verbosity command, and makes the two use
a common callback function.

This also adds a new entry into the release notes with different
formatting (including the priority and issue number)

Change-Id: I088f5418a58542ac0d7f17f9d696eafa85d2eac9
Reviewed-on: http://review.couchbase.org/40064
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>