Add check to ensure reset binding is only ever a switch
Tidied up pairing sequence. Removed some unnecessary unsolicited messages to reduce TX time. Handle ReTx more intelligently by waking device up first and then sending message.
Fix a couple of small issues with credit handling.

1) Initialise last send date with time that binding is instantiated. Avoids assuming last transmit was at the beginning of time and thus allocating maximum credit
2) Change max credit to be inline with culfw.
3) If there isn't enough credit and a message is queued then set it to slow send as device will probably have fallen asleep by the time we ACK.
4) Fix calculation of time until TX to take account of if it is fast or slow transmit method
5) Fix sequence removal/run ordering for acks
6) Ensure we remove message from sequence register after processing.