also do concurrent bucket deletion test without keeping extra ref
also do concurrent bucket deletion test without keeping extra ref

Change-Id: I33a5cb4fca19f5a38e5934f02f424c46f424c5a4
Reviewed-on: http://review.couchbase.org/13912
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

couchbase/bucket_engine,membase/bucket_engine,membase/bucket_engine,couchbase/bucket_engine,membase/bucket_engine,couchbase/bucket_engine,couchbase/bucket_engine,membase/bucket_engine