moving flush to bucket manager.

Change-Id: Icc832a0267c826eaa66bca59fd9575147e930cb0
Reviewed-on: http://review.couchbase.org/40336
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>
Tested-by: Michael Nitschinger <michael.nitschinger@couchbase.com>