Started using timeit for measuring elapsed time

[VitalyAssurance]

There is some evidence based on Datadog traces that backoff library does not calculate elapsed time correctly. For example, there are occurrences where extra retries are made even though the elapsed time exceeds specified max_time parameter for backoff.on_exception decorator. I suspect the problem is due to system clock being adjusted for time drift, or a similar issue.

I propose to use timeit instead of datetime.datetime.now() for measuring elapsed time.