Path MTU Discovery optional enhancements

[WesleyRosenblum]

• [ ] Track changes in a path (RTT Estimate changing) to determine if that path should be probed
  • It is RECOMMENDED that methods are robust to path changes that could have occurred since the path characteristics were last confirmed and to the possibility of inconsistent path information being received. RFC8899 §3
  • It is RECOMMENDED that methods determine whether the path has changed since it last measured the path. This can help determine when to probe the path again. RFC8899 §3
• [ ] Add a cache provider to the controller for caching PMTU values between connections
  • Possibly implement this as a Provider to allow customers to have their own PMTU cache that they can populate with information outside of s2n-quic
• [ ] Update PMTU for each path that shares destination IP
  • If PLPMTUD updates the MTU for a particular path, all Packetization Layer sessions that share the path representation (as described in Section 5.2) SHOULD be notified to make use of the new MTU. RFC8899 §3
• [ ] Update the max datagram size in the mtu::Controller based on the max_udp_payload_size transport parameter