Add classes for Task and SharedTask in threadpool

[ypatia]

Currently ThreadPool::Task is a typedef to std:: future<Status>. This PR:

1. Replaces this with a full class that imposes things like wait for our async processes to go through our ThreadPool's wait method. In that way, we never use a std::future's wait method directly and we avoid possible deadlocks because only the ThreadPool's wait will yield.
2. Adjusts the structure and relationship of tasks and the threadpool so that the caller can rely on the task's internal functions and doesn't need to keep track of the relationship between the ThreadPool and each task manually throughout the codebase.
3. Adds a new first class citizen of our ThreadPool: SharedTask, that is encapsulating std::shared_future which allows multiple threads to wait on an async operation result to become available. This is needed for the upcoming work on parallelizing IO and compute operations in the codebase even further.

This is heavily influenced from similar work by @Shelnutt2.

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TYPE: IMPROVEMENT
DESC: Add classes for Task and SharedTask in threadpool