Found out that ev_timer watchers have a void* data that allows
currying information to the libev callbacks! That means I could
stick the Command* in there and completely get rid of the global
timer_callbacks map that was needed before.
Additionally, we can move ev_timer into the Command object instead
of separately allocating it on the heap. Simplifies management.
Implemented a synchronous loop speed test based on the latest
and greatest command_blocking(). Happy to see it is around
30k/s, which is 100% network delays.
