timers: refactor timers

Consolidates the implementation of regular and internal (_unrefActive)
timers.

Also includes a couple optimizations:
- Isolates the try/catch from listOnTimeout() in a new tryOnTimeout().
- Uses a TimersList constructor as the base for linkedlists.

Additionally includes other cleanup and clarification, such as a rename
of "Timer" to "TimerWrap".

PR-URL: #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>