REP-6875 Augment /progress API endpoint

[FGasper]

The current /progress API lacks most of the useful details that the logs now expose. This changeset addresses some of those.

Specifically, this adds:

• per-generation stats:
  • time elapsed (in Go duration format)
  • number of docs compared
  • total # of docs expected
  • source bytes compared
  • (for generation 0) total source bytes expected
  • number of documents being rechecked (and why: from a mismatch, or a change event)
  • number of mismatches found
• source & destination change reader stats:
  • events per second
  • last resume time (i.e., the time to resume from if we restart)
  • last-seen cluster time
  • buffer saturation

These will allow external tooling to report verifier’s progress in useful detail.