Repeat calls to methods using IRQ causes lockup

[CraigBurden]

Repeated calls to the .request(LoRa.RX_CONTINUOUS) method causes main thread to lockup.
This is because of the use of the following on ln551 in SX127x.py:

if isinstance(self._monitoring, Thread):
                self._monitoring.join()

The monitoring thread has no way to exit and by default .join() is a blocking call. The timeout passed to the monitor_continuous method is ~1000s, so the main thread will block until this timeout is reached. To fix this you need to provide a way for the monitor_continuous method to exit when requested to do so. This should be done either with a ThreadEvent that is used to break out of the while loop or simply a class attribute to do the same.

I have not yet tested this, but I assume that this issue will occur anytime that .request() or .endPacket() is called anything other than the first time as the same lines exist on ln474.