Make modem more reliable by enforcing CMGL, timeouts

- We now enforce the use of CMGL, primarily because polling the modem
  every once in a while helps us check it's still alive.
  (This has led to at least one incident of messages being delayed!)
- In addition, we add timeouts to all of our modem calls, and knife
  the modem connection in the side if it fails to respond within 2s.