timers: fix not to close reused timer handle

The timer handle is reused when it is unrefed in order to avoid new
callback in beforeExit(#3407). If it is unrefed within a setInterval
callback, the reused timer handle is closed so that setInterval no
longer keep working. This fix does not close the handle in case of
setInterval.

PR-URL: https://github.com/nodejs/node/pull/11646


Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>