Fix memory leak in dcpdrain
Fix memory leak in dcpdrain

Keep track of the allocated bufferevent structures and delete
them when the program terminates.

Change-Id: Ic657cb0cd8f98058d7c21d03063cbf82931d3ba9
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/133361
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

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