Resyncing the state for a room follows the following algorithm:

• Find a batch of events with partial state. We choose the first 100 events ordered by arrival time. For each event in the batch:
  • Skip if we have prev_events with partial state
  • Request the state at any completely missing prev_events
  • Resolve the state across the prev_events and persist

It's possible for that to get into a loop, where we pull out the first 100 events, fail to make progress on any of them, and repeat.

part of #12646