This change fixes the bug described below:
if a file is removed while the local backend List() runs,
the call will flag an accounting error.
The bug manifests itself if local backend is the Sync target
due to intrinsic concurrency.
The odds to hit this bug depend on --checkers and --transfers.
Chunker over local backend is affected even more because
updating a composite object with a smaller size content
translates into removing chunks on the underlying file system
and involves a number of List() calls.
4680c0776d