Reclaim FFmpeg thread resources when paused (maybe?)

[hbomb79]

Open to debate whether or not this makes sense, but it's reasonable to assume a user may pause a transcode in order to get a different transcode to start, however currently the 'thread resources' are claimed when the transcode task begins, and they're only released when the task completes. This includes when the task is successful, cancelled, or troubled.

However, we may want to consider this behavior for paused transcodes too. It's not as trivial as the other three states as those don't need to actually keep any state around. A pause needs to carefully dance with the service to properly release threads on suspension and re-acquire threads before resuming (which could mean the task waits for many minutes/hours).
Currently the transcode service simply does not allow for this type of behavior, but anything is possible... it's just whether the complexity is worth it.