Prepare the transition to std::mutex.

Instead of keeping all open outputs locked, let ClearTailChunk() stall
playback until MultipleOutputs::CheckPipe() has updated the MusicPipe.

With the new command, the decision to pause or close the output moves
into the output thread.

Prepare for adding another Cond attribute.

These attributes are printed in the "outputs" response, and the new
command "outputset" allows setting new values.

No attributes are currently implemented.

Prepare to add an abstract class AudioOutput, to be implemented by
plugins, to get rid of the C-style vtable.

struct AudioOutput should not know or care about multi-threading.

This allows removing the "base.pause" check from the JACK output
plugin, and will allow removing similar accesses from other plugins.