MB-23591: Re-instate isBucketDeletion check for flusher

MB-23591: Re-instate isBucketDeletion check for flusher

The VBDeleteTask maybe scheduled to run, and isBucketDeletion
flag indicates if so, thus the flusher still needs to check
it to ensure no writes occur to a file that is about to be
deleted.

Change-Id: I746eb372f35c9b838dfab2ee7ac2b6c59606e94a
Reviewed-on: http://review.couchbase.org/76186
Well-Formed: Build Bot <build@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: David Haikney <david.haikney@couchbase.com>
Reviewed-by: Dave Rigby <daver@couchbase.com>