Lightweight thread refactoring

[geeksville]

I'm cleaning up the threading in the app. This will allow a few things:

• Simpler
• Less FreeRTOS dependent (important for the port to Apache MyNewt - for low end NRF52s) and portduino
• Lower power consumption because the top level arduino loop() will be sleeping typically for 100s to thousands of msecs
• Lower latency - because we will be able to instantly wake for ble, button press or lora rx. Because there will no longer be the nasty 10ms sleep we had in loop() to save power. Instead there is a new InterruptableDelay class which allows sleeping and instantly waking if needed.