Draft: Jack support

[ylm]

Initial effort to integrate JACK into ORCA since portmidi doesn't support it.
A forseeable issue is the use of jack ringbuffer. It doesn't use memory barriers which are required by some architectures.
Open for discussion. Plan to have dynamic port creation too.

[Krasjet]

JACK's ringbuffer, unfortunately, is a problem. I opened an issue about an overbound read bug a while ago. Raspberry Pi (ARM architecture) is notably affected by it:

https://www.youtube.com/watch?v=dLdTDMsz7tg

but this bug is extremely rare. I hope it gets fixed soon.

zix's ringbuffer implementation handles memory ordering properly. Maybe it can be used as a temporary solution. Though notice that JACK's MIDI itself internally uses ringbuffer, so this problem has to be fixed in JACK.

[ylm]

After using this branch for a bit, there's an issue in how the midi events gets timestamped, so I need to get that ironed out first. An alternative would be to swap portmidi with RtMidi which now has a C interface and support many more backends (JACK being one of them).

[plexus]

@ylm did that last commit fix the issue you mentioned in your previous comment? I've been running this branch and so far haven't encountered any issues.