cc @davidtorres

The previous implementation modifies a HashMap while iterating through
its key set, causeing unpredictable iteration behavior.
This might be the reason our tests intermittently deadlock.

The new implementation uses a PriorityQueue.
The time complexity is O(M * log N),
where M is the number of expirations before the cut over time
and N is the total number of expirations.
I am not sure how this compares to O(N) intended in the previous
implementation.
If required, O(N) is also possible using an ArrayList.

Unfortunately, a new failure has emerged.
Instead of deadlocking, testModifyAckDeadline intermittently fails.
Maybe

• I have fixed the old bug and created a new one,
• I have fixed the old bug that was masking another one,
• The deadlock wasn't caused by the iteration. Now the tests just fail
  before they could deadlock,
  or some combination thereof.
  The incorrect iteration should be fixed regardless.