This is two-fold:
- "percent complete" over the entire sync (including two-phase syncs
  with text-only followed by photo sync) - to be used in a UI as progress
  indicator
- "add/updated/removed" contacts while the sync runs - to be used by
  other components instead of polling for changes

Both notifications need to be sent out at a smooth rate, neither too often
(overhead) nor too infrequently (which negates the purpose). Once per second?