timers: clean up for readability

Remove micro-optimizations that no longer yield any benefits,
restructure timers & immediates to be a bit more straightforward.

Adjust timers benchmarks to run long enough to offer meaningful data.

PR-URL: https://github.com/nodejs/node/pull/17279


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>