expire subsystem should be refactored for scale

[gdamore]

The single most painful and complex part of NNG's bowels is the asynchronous expiration system. It also puts the most stress on the system for lock contention.

What perhaps we should do is build on the idea of aio completion lists, and move from the one monolithic expire queue to individual subsystem queue (or better a queue per socket!), and to place the aios on an expiration list for that socket that is controlled by the subsystem lock.

This would allow for much simpler cancellation & completion.

We need to make sure we are sensible about linear search times with a single list. Perhaps it's time for NNG to get a minheap.

This is probably a substantial refactoring.