Pause unsynced clock when no sources are available

[toots]

There's a gap in our clock implementation where unsynced clocks will keep spinning when no sources are available which causes a lot of issues with CPU burn and livelock.

Typical use-case would be a full unsynced file conversion. After the output goes down, shutdown can be called and the program properly exists.

We should find a way to pause the clock when all sources are unavailable and resume it when this changes back. This gets complicated because some source may currently need an action from the clock to be ready again.