Add peek/pop code to the scheduler loop

[ferruzzi]

In the scheduler loop add “check for DeadlineAlert misses” which peeks at the earliest Deadline and, if it has passed, then call a callback handler.

The callback handler should peek at the earliest Deadline, if it has passed then queue the callback, and recurse until there are no past-due Deadlines..

When working on this one, the PR should include benchmarking to show the exact impact of the extra workload being added in the hot loop.