mcserver: request wwant() if not all data has been flushed

Change-Id: I9c6a545d0e2c4d0e347a8b9bf1a6e8a313999d64
Reviewed-on: http://review.couchbase.org/37565
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Brett Lawson <brett19@gmail.com>