refactor!: Major code cleanup for clearer state

[JR-Morgan]

Major refactor

Code cleanup, I've tried to make the state handling clearer by wrapping the worker state in a new Worker class.
Cleaner code around reset of state
More proper disposal logic (still not perfect)
Breaking change around DoWork function now returns a Task object.

Non-async function should simply return a Task.CompletedTask
async functions can now await

TODO: check with Adam, we may actually prefer using an async void pattern here. The root task isn't awaited, so it's not like we need to keep the await chain... but it feels weird to have a Task that wraps an async void

[adamhathcock]

Task.Factory.StartNew()

[adamhathcock]

No try/catch or other error handling for this task? Unobserved task exceptions 😬

[JR-Morgan]

It's important that the tasks don't start within this SolveInstance.
They get started all together in PostSolveInstance

That way, it it guaranteed that a task never calls Done while we are still spinning up workers. That would break the components state tracking.

[JR-Morgan]

Perhaps there's alternatives with storing Func<Task>, but for this PR I wanted to keep minimal changes

[adamhathcock]

We probably want some basic error handling and feed back. I don't think we should be delegating this to the caller, at least for cancellation exceptions.

Good clean up though, much nicer.

[adamhathcock]

No try/catch or other error handling for this task? Unobserved task exceptions 😬

[JR-Morgan]

No error handling, because this component cannot really handle the errors.
Not ideal, I would really like to refactor a bit how cancellation is handled. But to keep these changes minimal, I've not changed the current pattern