DVDThread: State save read requests without result data.

[jordan-woyak]

This completes two TODOs in DVDThread.

Rather than waiting for read requests to complete, all requests are now state saved without their results.

I added some additional features to WorkQueueThread to accomplish this.
A separate WaitableSPSCQueue command queue eliminates the "run_thread" and "skip_work" flags and allows additional tasks like gathering all the items while the thread is running.

I also changed result data from std::vector to Common::UniqueBuffer to avoid the zero-initialization.

In my testing, there are usually zero pending requests, and this isn't a huge benefit, but here it is anyway.
Maybe someone knows a particular game that's DVD-read heavy?

[JosJuice]

Maybe someone knows a particular game that's DVD-read heavy?

I guess you could try a DTK game? One that loads data while still using DTK would be ideal, as that's the only way you can get more than one command in the queue at the time IIRC. I suppose Super Monkey Ball 1 and 2 fit the bill - maybe there are others too.

[jordan-woyak]

Alright, with and without this PR, DVDThread::DoState generally takes less than 2us to handle zero pending requests (which is the typical situation).

In Super Monkey Ball 2, when mashing the the Save State button during game play I could occasionally see 3 pending requests.
Master's DoState would take around 12us to handle this.
In this PR, however, it was still around 2us.

When mashing Save State during boot I could see quite a bit more pending requests, around 15 or so.

In master, 15 pending requests could take more than 150us.
In this PR, the same situation was usually under 3us, and never more than 10us.

So yes, I suppose this PR makes things faster, in atypical situations.