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.

Several hours of multithreaded debugging to find a nasty extra
ampersand. command_blocking() now returns a Command object with
the value and status accessible through methods.

Added the option to free memory or not for Command objects when
command() is called. If free_memory = true, then the Command
object is freed after the callback or error callback returns.
If free_memory = false, then the user must call cmd->free().

To implement this, we have to pass in a blank function to the
freeObject entry in the hiredis redisReader, because otherwise
it automatically frees memory. This was not cool for the blocking
case.

Implemented error callbacks for command(). Now, either the success
or error callbacks are guaranteed to return for any command. Added
constants for error types.

Many improvements and fixes for memory management. Command objects
keep track of pending replies and delete themselves when completed.

Changed ReplyT value to remove the const and reference requirements.
Now, you do command<ReplyT> and the callback should take a
const ReplyT&, whatever the type is. Makes things more uniform.

Renamed CommandAsync to Command to make things simpler, and
refactored into its own file.

command() now returns a pointer to the created Command object,
which the client can pass into cancel() to stop any delayed
or repeating calls. This is very important for an asynchronous
client.

Uses libev timer watchers to queue commands on a regular
interval. Very fast. Still need to handle deactivation of
timers from the user side.