cb::unique_item_ptr should fill in the owning engine
cb::unique_item_ptr should fill in the owning engine

When we're moving to bufferevent the unique_item_ptr is
moved directly over to bufferevent and we need to keep
the knowledge of the owning engine in order to correctly
release the item.

Change-Id: I5af8c06bdb42d9c80e2c0b3b7d23ac2db0f761a0
Reviewed-on: http://review.couchbase.org/108716
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Jim Walker <1cd02e31b43620d7c664e038ca42a060d61727b9@couchbase.com>

daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine,daverigby/kv_engine