Future Checks don't always get disabled

[corbob]

While working to reproduce and debug chocolatey/choco#2289, @vexx32 found this handy note about Timer.Dispose:

Callbacks can occur after the Dispose() method overload has been called, because the timer queues callbacks for execution by thread pool threads. You can use the Dispose(WaitHandle) method overload to wait until all callbacks have completed.

If we are interpretting this correctly, what this is saying is that even though we dispose of the timer in DisableFutureChecks(), we don't guarantee that it'll never fire.

Further, the remarks for Timer.Change state:

If dueTime is Timeout.Infinite, the callback method is never invoked; the timer is disabled, but can be re-enabled by calling Change and specifying a positive value for dueTime.

If period is zero (0) or Timeout.Infinite, and dueTime is not Timeout.Infinite, the callback method is invoked once; the periodic behavior of the timer is disabled, but can be re-enabled by calling Change and specifying a positive value for period.

Which to me says that to disable the Timer, you can Change it with a dueTime of Timeout.Infinite and a period of Timeout.Infinite.