Should help #377 a little

Fixes #337

Since JACK1 runs the rt and non-rt events on the same thread,
blocking in the non-rt events can still cause xruns.
So now on non-rt JACK events we simply save the event to handle
later on a separate thread.
This was never an issue with JACK2 and the original code could be
kept, but I don't want to have 2 different implementations for
each JACK version (possibly leaving one untested).

Also some needed cleanup

Closes #269

Closes #262

This is going to be needed for rtaudio/juce patchbay modes,
but it's also nice to have for JACK.