Implement container task queue

The container task queue is an in-memory iterator/cursor to get tasks that are waiting for a pod to be allocated to them sorted by when they were scheduled. This iterator does not need to take into account the max parallel pod request limit since it is only responsible for providing tasks in the order that they were scheduled; the Amboy job population logic will handle how to respect the max parallel pod request limit.

For compatibility with the already-existing meaning of a scheduleable task, the filters in this scheduleable tasks query should still apply in order to include only those tasks that are undispatched(i.e. but activated and scheduled) while filtering out disabled tasks (i.e. those with negative priority) and those with unmet dependencies (unless they're overridden).