We need to know the most recent scan id we have actually completed. This is to
handle the case where a guest disconnects when we're in the middle of streaming
worktree entries to them. When they reconnect, they need to report a scan_id
from before we started streaming the entries, because we have no record of when
the stream was interrupted.

Next failure:
SEED=5051 ITERATIONS=1 OPERATIONS=200 cargo test --release --package=collab random -- --nocapture