fixed reset of delete_bucket connection's engine handle
fixed reset of delete_bucket connection's engine handle

We need to do that via set_engine_handle so that refcount is dropped.

Change-Id: I258342db992f58a2bc1115e5c0b9518d3bdb6ebf
Reviewed-on: http://review.couchbase.org/13875
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Reviewed-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>
Tested-by: Chiyoung Seo <8b279095f11c26cc21045fda5a3e51debb005aa9@gmail.com>

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