DISCONNECT duration 16-bit field does not allow long enough to sleep

Assuming the time unit of a DISCONNECTED sleep session is SECONDS, per the 1.2 specification, this only allows a sleeping period of 18 hours 12 minutes and 15 seconds. This is not enough for real-world LPWAN devices. We need to somehow accommodate more space for larger durations OR change the time unit of the duration field to be minutes (which would allow for ~45 days sleeping).