timers: assure setTimeout callback only runs once

Calling this.unref() during the callback of SetTimeout caused the
callback to get executed twice because unref() didn't expect to be
called during that time and did not stop the ref()ed Timeout but
did start a new timer. This commit prevents the new timer creation
when the callback was already called.

Fixes: https://github.com/iojs/io.js/issues/1191


Reviewed-by: Trevor Norris <trev.norris@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
PR-URL: https://github.com/iojs/io.js/pull/1231