Fixed rapid network acquisition

[martinichka]

Dear maintainers.

This PR is accompanying the Lpd Community case: 65290

The LoRaWAN Basics modem supports the US915 rapid network acquisition as described by the regional parameters (on join random selecting one channel from the first band, on no response randomly selecting a channel from the second band etc). This is very important for US915 networks that have only an 8 channel network.

Although the algorithm works correctly, the timing is not correct. Some regions are duty cycle limited (e.g. EU868) and others are dwell-time limited (e.g. US915). But it seems that the US915 join process use duty cycle limitation as well. This leads into a very slow join process.

We would be happy to add information if needed. Let me know.