Iterate over select vbs only for flushOneDeleteAll

- Iterate over only the vbuckets that belong to the
particular shard when flushOneDeleteAll invoked.

Change-Id: Ia990dfef4596c3aa101209e4e3ce9482be548aea
Reviewed-on: http://review.couchbase.org/34653
Reviewed-by: David Liao <david.liao@couchbase.com>
Tested-by: David Liao <david.liao@couchbase.com>