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/collab/migrations.sqlite/20221109000000_test_schema.sql

@@ -57,7 +57,7 @@ CREATE TABLE "worktrees" (
     "abs_path" VARCHAR NOT NULL,
     "visible" BOOL NOT NULL,
     "scan_id" INTEGER NOT NULL,
-    "is_complete" BOOL NOT NULL,
+    "completed_scan_id" INTEGER NOT NULL,
     PRIMARY KEY(project_id, id)
 );
 CREATE INDEX "index_worktrees_on_project_id" ON "worktrees" ("project_id");