MB-9404 FlushAll should be invoked across all the shards

FlushAll(i.e., database reset) was only invoked with a single
shard before. This can result in an inconsistent state and
doesn't clean up the underlying storage states.

Change-Id: I6485e22e6234c488eeb87e648c42f1468d863648
Reviewed-on: http://review.couchbase.org/29707
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>