[mock_uss] Better slow update check

[the-glu]

Follow #1333

Improve slow update check in mock_uss by doing a proper slow update check.

This also make notification coming faster instead of having arbitrarily in the middle of the flight.

Idea is the following:

• Compute average update rate with a moving window of 3 seconds.
• Check for % of update rate bellow the required rate with a moving window of 10 (over the averages update rates)

Notifications are created for each "non-compliant bucket", and placed in time at the end of the bucket, making them more realistic. However, windows spans are quite arbitrary.

[BenjaminPelletier]

Since this value (10s window) is used in 4 places, it's important to understand exactly what it is, and we may want to change it, let's make it a constant defined and documented once and then used in all applicable places. We probably also want to type-annotate it as an int since a float value would not work even though conceptually it seems like that would be ok.

[BenjaminPelletier]

Since we've already quantized to buckets, wouldn't it be simpler to do this computation in that quantized frame?

Suggested change

	current = start + datetime.timedelta(seconds=2)
	while current < end:
	bucket = int((current - start).total_seconds())
	rates.append(
	(buckets[bucket] + buckets[bucket - 1] + buckets[bucket - 2]) / 3.0
	)
	current += datetime.timedelta(seconds=1)
	last_bucket = int((end - start).total_seconds())
	bucket = 2
	while bucket <= last_bucket:
	rates.append(
	(buckets[bucket] + buckets[bucket - 1] + buckets[bucket - 2]) / 3.0
	)
	bucket += 1