timers: reuse timer in `setTimeout().unref()`

Instead of creating new timer - reuse the timer from the freelist. This
won't make the freelist timer active for the duration of `uv_close()`,
and will let the event-loop exit properly.

Fix: #1264
PR-URL: https://github.com/nodejs/node/pull/3407


Reviewed-By: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>