DNM: Task Queue Compacting and Task Ordering

Background
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This pull request enables `DynamoScheduler` to perform two important steps in task management -- to *compact the task queue*, and to *reorder tasks in queue*. These two steps are done right before `ISchedulerThread` picks up the next task for processing. Which means many `AsyncTask` can be scheduled before `ISchedulerThread` returns for more task, making queue compacting more effective (i.e. there might be more tasks for compacting when `ISchedulerThread` revitis the queue).

##### Compacting task queue
Compacting task queue means removing redundant tasks on the queue. `AsyncTask.Compare` method is introduced to allow comparison between two `AsyncTask` objects. The return value from this call helps `DynamoScheduler` determines if either of the tasks should be discarded, or both should be kept. Derived classes override this method to provide type specific decision.

##### Reordering tasks
Take reordering is done right after task queue has been compacted. In this step, the remaining tasks in queue are sorted based on their *priority*. `DynamoScheduler` relies on the return value of `AsyncTask.WeighAgainst` method in determining the relative priority of any two given tasks. Derived `AsyncTask` classes override this method to provide type specific prioritization.

Unit test
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Two mock `AsyncTask` classes are introduced to ensure these two task management steps do not regress: `PrioritizedAsyncTask` and `InconsequentialAsyncTask`. Please see source code documentation for more details on their roles. A new test case `TestTaskQueuePreProcessing` is written in conjunction with these two new classes to test the new task management functionality.

Notes to reviewer
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Hi @lukechurch, thanks for looking. The actual `AsyncTask` derived classes are not overriding `CompareCore` and `WeighAgainstCore` methods yet. So even queue compacting and task ordering are already enabled, they do not have any effect on the way tasks are processed today. I will be back to address your review comments soon as I can.
`TaskMergeInstruction` is what I have been looking for, thanks!