Design more robust API for applications

[janakj]

The AT command set inspired by Murata is pretty low-level. Most (if not all) commands can fail if invoked at the wrong time, e.g., while the modem is busy transmitting or retransmitting an uplink. This is generally PITA for the application. Handling this reliably individually at application level is not realistic.

We may need to design a more high-level API which will handle most common failure modes internally within the LoRa modem firmware. This could be implemented, e.g., by queuing AT commands submitted by the application until the MAC is in the correct state.