Improve iterator ETA

[RunDevelopment]

Changes:

• Use a mix between an exponential and harmonic falloff to calculate a weighted average that favors recent samples. I also capped it at 100 samples to (1) ignore very old samples and (2) keep the computational overhead to a minimum.
  Fixes Video frame iterator ETA not recalculating when load changes #2324
• Use time.monotonic() to measure time.
• Move logic for keeping track of time into _IterationTimer class.
• Account for pauses when measuring time.
• Don't let ETA continue to count down and show "Paused" instead when the execution is paused.