Fake hangups from backend DCBs cause worker messages to be emitted

When a MariaDBBackend DCB is being closed, it emits a fake hangup event on the client DCB. The current code only has a fast path for events placed on the current DCB which causes a queued message to be posted on the worker itself every time a client connection is closed. In extreme cases this can cause lost messages if a large amount of backend connections close at the same time.