Allow PeriodicTimer to have an infinite timeout (#82986)

Allow PeriodicTimer to have an infinite timeout (#82986)

* Allow PeriodicTimer to have an infinite timeout

When PeriodicTimer was introduced, we prohibited infinite timeouts as they made no sense: you couldn't change the period after construction, so creating a PeriodicTimer with an infinite timeout meant creating an instance that would never tick.  But now that we've added a Period property with a setter that allows changing the period, infinite timeouts do make sense: as with Timer, they can be used as a mechanism to stop and start the timer, e.g. creating the PeriodicTimer with infinite and only in response to some later action updating its Period to start it.

* Update PeriodicTimer.cs