Handle beat-tracking for starts/stops

[oubiwann]

Only one start was allowed; let's allow for any number of starts/stops.

Tasks:

• Come up with good name and update the data structures
• Refactor transport into an actual abstraction (and with its own constructor)
• Transport could be used anywhere, so put it in um.transport
• Update all transport-related logic to use new abstraction
• Put ms <-> s time conversions into a function
• Provide some basic/common time functions for better reuse
• Fix padding of H:M:S in formatted times to HH:MM:SS

Part of:

• Support external clock-based orchestration #42

[oubiwann]

Things are moving in a different direction -- closing this one.