Drime's folder rename returns success immediately but the children
listing of the renamed folder briefly returns empty before settling,
which made VFS see an empty directory right after a successful rename
and broke subsequent operations on its contents.
After a successful DirMove, poll the renamed folder's listing until it
shows the pre-rename child count, with a 30s timeout and exponential
backoff. The backend integration test already tolerated this via its
own list-consistency retries; this brings VFS into line.
Fixes#9450
Nick Craig-Wood
387be8d8c0