Eliminate the separate timer

[gdamore]

The timer logic we have in src/core/timer.c could probably go away now. The main timeouts we have are using aios and their builtin timers, and it seems that the timer.c code is now duplicate; it also forces yet another thread to be created.

[gdamore]

Just a note here, the structure of aios and cancellation of them seems to be somewhat rough, in that cancellation of the aio may cost quite a bit, whereas with timers we can simply deschedule the timer. The REQ protocol makes use of this to reduce the cost of what would otherwise be a fairly frequent code path.

One thing we might look at doing is scheduling a receive from the REQ, even before the client has issued it, and then having that timeout. That would allow us to use the same AIO for both purposes.