Rate limit new quic connections

[sukunrt]

trafficstars

quic-go provides the ability to gate connections with the config.GetConfigForClient callback and to verify the source address of packets with transport.VerifySourceAddress callback.

We should GetConfigForClient this to rate limit new connections and plug the connection gater and the conn limiter here so that users can do their custom limiting.

	// GetConfigForClient is called for incoming connections.
	// If the error is not nil, the connection attempt is refused.
	GetConfigForClient func(info *ClientHelloInfo) (*Config, error)

[rose2221]

would like to work on this