When doing a bucket flush only send reset to mccouch once

Since we have multiple shards in that each have a kvstore we should
only allow one of the shards to send a flush notification to
mccouch.

Change-Id: I62a4fe0b954f5a004f1c16bfbdb57b7fabfa04c8
Reviewed-on: http://review.couchbase.org/32796
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>