UDP could do a better job of queueing

[gdamore]

We're seeing some "performance" problems that look like lots of dropped messages.

I think this may be a consequence of the design for UDP being based on the same design we used for TCP, but unlike TCP, UDP can benefit from from the fact that each message stands alone, and does not depend on the messages before or after.

Consequently, I suspect we could use a batch of aios to perform fast turnarounds on the receive operations. Some care would be needed to ensure callbacks are ordered properly on the receive side however. (In order to avoid reordering based drops.)

Also, linux has the recvmmsg and sendmmsg calls which can reduce the number of system calls we perform when doing these operations. That's likely to be worth exploring as well.