timers: greatly improve code comments

Describes the How and Why of the timers implementation, as well as
adding comments in spots that should allow for an easier understanding
about what is going on.

The timers implementation is very efficient, at a cost.
That cost is readable understandability, and this aims to improve that.

PR-URL: https://github.com/nodejs/node/pull/4007


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>