timers: add internal [@@ refresh()] function

Hidden via a symbol because I'm unsure exactly what the API should look
like in the end.

Removes the need to use _unrefActive for efficiently refreshing
timeouts.
It still uses it under the hood but that could be replaced with
insert() directly if it were in the same file.

PR-URL: https://github.com/nodejs/node/pull/18065


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>