invalid retirement of connection ID when receiving a duplicate NCID frame for a CID that's in use

[marten-seemann]

When we receive a duplicate NEW_CONNECTION_ID for the the connection ID that's currently in use, we immediately send a RETIRE_CONNECTION_ID frame for that connection ID, without actually retiring it.

That means that this connection ID is retired in a packet that uses the same connection ID. This is a protocol violation. More importantly, since we never actually retire the connection ID, packets will end up getting dropped as soon as the peer drops the association between that connection ID and the session (which happens after 5 seconds in quic-go).