lorawan: native: serialize mutable API state

Several public APIs update or consume shared MAC state while the engine
thread is processing join and send requests.

Use a single API mutex for join, send, and mutable setters so only one
public operation can touch the native LoRaWAN state at a time. Also
reject region changes after the stack has started.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>