timers: refactor timer list processing

Instead of using kOnTimeout index to track a special list
processing function, just pass in a function to C++ at
startup that executes all handles and determines which
function to call.

This change improves the performance of unpooled timeouts
by roughly 20%, as well as makes the unref/ref processing
easier to follow.

PR-URL: https://github.com/nodejs/node/pull/18582


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>