Panic in connWatcher when connTable is stopped

[StefanKopieczek]

If connTable.Stop() is called when at least one connection in the table has expired, the connWatcher panics when it receives the stop message from the connTable.

Cause is that when the connWatcher nil-s out its connection both on expiry and when it is stopped; the second nil causes a nil dereference.

Could just do a nil check here, but there's a wider issue that the connWatcher should be GC'd on socket expiry, and it currently isn't. We can't just remove it from the table inline, because we're running on a separate goroutine so this could cause races. So we'll need a dedicated goroutine for the connTable which serializes conn updates and expiries.

[rynorris]

Can this now be closed?