Peter M. Groen / redox

Added Command<nullptr_t> which successfully returns NIL
replies from Redis, with REDOX_OK. Also renamed constants
from REDISX_ -> REDOX_.

Moved a bunch of code from redox.cpp into command.cpp, where
it logically makes more sense.

Move as much as possible into the .cpp file, and clean things up.

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.