Additional assertions that connection buffers have been freed

Change-Id: I13a88fc3f72e26251e31b7df9bc4689bab38f385
Reviewed-on: http://review.couchbase.org/39595
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>