Support external clock-based orchestration

[oubiwann]

Part of epic:

• 38

Tasks:

• [x] Allow changes to time signature while running
• [x] Change single start/stop data to a list of starts/stops
  • [x] update the map for tracks to remove started_at and stopped_at
  • [x] add new key transport that's an empty list by default
  • [x] every call to start-track and stop-track will append to the transport field, e.g., #(stop ,now))
• [ ] #46
• [ ] Receive start/stop messages from midiserver
  • [ ] https://github.com/ut-proj/midiserver/issues/29
  • [ ] update transport with timestamps when start/stop msgs received from midiserver
• [ ] Support callbacks based upon changes in timeclock data (e.g., sending MIDI messages, leaving room/time for message transmission -- should be delivered before the target beat, but after the beat prior to the target beat), e.g. at:
  • [ ] a given beat by absolute count
  • [ ] a given beat for a given measure
  • [ ] next beat
  • [ ] next nth beat of the next measure
  • [ ] next nth beat of the next nth measure