Lots of `Rotating session for room` lines on startup

When resuming a session, we log thousands of lines like:

```
Rotating session for room !:matrix.org due to member leaving the room
```

This is an artifact of matrix-org/matrix-js-sdk#5231, together with the js-sdk's "sync accumulator". When we resume a session, the js-sdk replays all state events in all rooms, which triggers the above logging.

It's relatively harmless, though certainly not great for performance. (The logging is less of a concern than the indexeddb operation we do for each line, since the absence of an active session is uncached)