Add general rate limit

[zpostfacto]

Currently connections on Steam that are relayed are rate limited by the relays. But we should probably also implement a generic packet rate limiter and add some configuration values so the application can tune this on a global or per-connection basis.

That way basic UDPv4 connections are not vulnerable to malicious peers.

The rate limit will be applied after the nonce has been checked, so random spam packets shouldn't trigger it, only hosts who have already connected.

[alexanderperrin]

Hi Fletcher, not sure if this is a fine place to ask, so please let me know if not, but do you have any information on what rate limits are used by the relays (SDR)?