proposition to fix #163

[klementc]

This pull request fixes issue #163

Proposed Changes

• Check nextTx is indeed a retransmission before assigning it to ensure we won't cancel another type of event
• Schedule a new Send instead of doSend to recheck if the channel is available or if another delay has to be applied
• Add debug log line at start of resetRetransmissionParameters

This does not break the tests, but tell me if it is a problem for another part of the LoRa code (I am still figuring out some of it)