Region payload limits not adjusted by TxParamSetupReq/dwell limits support

[Bwooce]

[Raised as an FYI to document my discovery, this can be closed as Won'tImplementRightNow :-) ]

The Payload limits are zero for slow DRs in AU915 and AS923.. This is correct and as per all the recent (incl. 1.1) region specs which say to start at the Dwell 1 limits and use DR2 (SF10/125) until MAC TxParamSetupReq is received with a dwell change (to 0). I'm not even sure TTNv2 sends this...v3 does now.

Hence there are two limits for AS923 and AU915 e.g. see LoRaMac-node)

This is a bit of a problem for me when porting TTN Mapper type code. I don't see any lmic stack variant supporting this, but SF12/SF11 are not usable in those two regions without it. I'm going to hack it in for now but this annoying dwell stuff seems to be specific to only these two regions so annoying to implement.

[matthijskooijman]

Thanks for documenting this. I'm not too familiar with those regions or the concept of "dwell", so I'm not going to do anything with this directly, but as always, patches welcome :-)