timers: fix timeout when added in timer's callback

When a timer is added in another timer's callback, its underlying timer
handle will be started with a timeout that is actually incorrect.

The reason is that  the value that represents the current time is not
updated between the time the original callback is called and the time
the added timer is processed by timers.listOnTimeout. That leads the
logic in timers.listOnTimeout to do an incorrect computation that makes
the added timer fire with a timeout of scheduledTimeout +
timeSpentInCallback.

This change fixes that and make timers scheduled within other timers'
callbacks fire as expected.

Fixes #9333 and #15447.

PR: #17203
PR-URL: https://github.com/joyent/node/pull/17203


Reviewed-By: Fedor Indutny <fedor@indutny.com>