REJOIN code and logic is pre-LoRaWAN, should come out

[terrillmoore]

All of the logic related to HDR_FTYPE_REJOIN is obsolete. That header type didn't make it into the 1.0.2 spec. Remove HDR_FTYPE_REJOIN, all uses of OP_REJOIN, the API LMIC_tryRejoin().

[terrillmoore]

It turns out, after studying the code some more, that OP_REJOIN is not obsolete, even though HDR_FTYPE_REJOIN is. In the LMIC code, a rejoin is a "make before break" join and is used for several purposes (particularly in ADR recovery when we've seen nothing for quite a while). So although we should change joins that are triggered by OP_REJOIN to use HDR_FTYP_JREQ, we should not remove the concept.

In addition, the rejoin request is sent using the datarate suggested by the LMIC normal transmission logic. It's not clear that this is right; I think join requests now should be sent at the network-defined rate. Review this when removing HDR_FTYPE_REJOIN.

[terrillmoore]

With the changes from #275, #276, #278, I think we're ready to use this properly.