UDP: faster close (in some rare situations)

[patterniha]

#4899 (comment)

[Fangliding]

那么多没合呢要不你还是消停会

[patterniha]

also, If this happens, we handle the new received packet incorrectly, because we create a new outbound connection but we ignore it !

so we should run conn.updateActivity() in udpWorker lock:

if conn, found := w.activeConn[id]; found && !conn.done.Done() {
		conn.updateActivity()
		return conn, true
}

[patterniha]

I managed to produce an example(with two interface) where udpConn is closed but connEntry is not closed, so this PR should be merged to fix this (rare) situation.