timers: improve setImmediate() performance

This commit avoids re-creating a new immediate queue object every
time the immediate queue is processed. Additionally, a few functions
are tweaked to make them inlineable.

These changes give ~6-7% boost in setImmediate() performance in the
existing setImmediate() benchmarks.

PR-URL: https://github.com/nodejs/node/pull/8655


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>