MB-100: loop phase waits until drained

Change-Id: I51d0ae1e5823a07a8d6dca637ee513f4dfebdca8
Reviewed-on: http://review.couchbase.org/25209
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>
Tested-by: Pavel Paulau <pavel.paulau@gmail.com>