Path MTU discovery about quinn

[Ralith]

On Linux, this will require setting the socket option IP_MTU_DISCOVER to IP_PMTUDISC_PROBE to set the "don't fragment" flag and bypass the kernel's built-in ICMP-based MTU estimation.

[djc]

Note that this is the exact language in draft-16:

To optimize bandwidth efficiency, endpoints SHOULD use Packetization Layer PMTU Discovery. Endpoints MAY use PMTU Discovery (PMTUDv4, PMTUDv6) for detecting the PMTU, setting the PMTU appropriately, and storing the result of previous PMTU determinations.

So, implementations MAY use both PMTUD and PLPMTUD.

[Ralith]

Once this is implemented, we should (as a separate feature) emit an event indicating when the maximum datagram frame size changes.