US compliance: DR decay doesn't cancel large packet tx

[terrillmoore]

As ADR or LMIC confirmed message retry reduces the data rate ("data rate decay") it is possible that we will cross a feasibilty boundary; a message might be feasible to send at DR1, but not at DR0. RWC5020A V1.170 test 12.3 checks this by sending a confirmed echo request, not ACKing, and watching what the device does. We should stop attempting the confirmed uplink.

The LMIC doesn't consider this, it just keeps retrying. This is very similar to #388, and cannot be handled by the app; and any fix that fixes this is likely to fix #388.