Introduce completed_scan_id to worktree

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

crates/call/src/room.rs

@@ -382,7 +382,7 @@ impl Room {
                                 let worktree = worktree.read(cx);
                                 proto::RejoinWorktree {
                                     id: worktree.id().to_proto(),
-                                    scan_id: worktree.scan_id() as u64,
+                                    scan_id: worktree.completed_scan_id() as u64,
                                 }
                             })
                             .collect(),