Compliance test / 64 channels: join by subband

[terrillmoore]

In the compliance test for US915, we are asked to rejoin. This can take a long time if 64 channels are enabled. (And we'd like to test this without changing the device settings for test.)

This would be better if we implemented the subband-by-subband scan for join request mentioned in the LoRa Alliance docs. It would also be better if we also restarted our probe in the last found subband (and focused a few probes in that area).

Once we fix the join-flood issue (#546), this will just be worst, because the time gets very long...

[terrillmoore]

The pre-compliance tests seem to pass anyway, although it slows them down somewhat. Low priority fix.