Support external clock-based orchestration

[oubiwann]

Part of epic:

• Add VCV modular synth support #38

Tasks:

• Allow changes to time signature while running
• Change single start/stop data to a list of starts/stops
  • update the map for tracks to remove started_at and stopped_at
  • add new key transport that's an empty list by default
  • every call to start-track and stop-track will append to the transport field, e.g., #(stop ,now))
• Set the BPM explicitly from LFE #46
• Receive start/stop messages from midiserver
  • Send start/stop messages in receive callbacks geomyidia/midiserver#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