Augment /progress API to report most of what logs report.

[FGasper]

This adds the following data points:

• change event stats (for src & dst):
  • events per second
  • latest cluster time
  • resume time (i.e., from the most recent resume token)
  • buffer saturation
• per-generation stats:
  • active worker threads
  • time elapsed
  • docs compared & total docs
  • bytes compared & (for gen0) total docs
  • total mismatches found
  • total rechecks enqueued