Use monotonic time

[wapiflapi]

Backoff uses timedelta.total_seconds(datetime.datetime.now() - start) to compute elapsed time which is used to figure out if max_time has been reached. This is flaky and doesn't work well when the system clock is being updated: when adjusting time from the network or adjusting to daylight saving time for example.

Instead monotonic time should be used, using for example https://docs.python.org/3/library/time.html#time.monotonic