handle_disconnect is not properly forwarded to ep_engine

There is something wrong in the way the notifications work in bucket_engine. ep-engine tries to register an on_disconnect callback during it's initialization phase, but it does not receive anyone. Please note that I'm using the default bucket (so it could be something special with that).

To verify this, just add a node to the system and do a rebalance. That should leave a ton of anon tap connections on the server...