fix: remove deadlock in multiPacketListener

[sbruens]

The multiPacketListener and virtualPacketConn are prone to deadlocks during concurrent ReadFrom() and Close() operations. A deadlock when closing connections concurrently with ongoing read operations is preventing the config from getting reloaded on receiving multiple SIGHUP signals.

[fortuna]

I think we should still check the if the readCh is closed, but otherwise LGTM