* filer.sync: show active chunk transfers when sync progress stalls
When the sync watermark is not advancing, print each in-progress chunk
transfer with its file path, bytes received so far, and current status
(downloading, uploading, or waiting with backoff duration). This helps
diagnose which files are blocking progress during replication.
Closes#8542
* filer.sync: include last error in stall diagnostics
* filer.sync: fix data races in ChunkTransferStatus
Add sync.RWMutex to ChunkTransferStatus and lock around all field
mutations in fetchAndWrite. ActiveTransfers now returns value copies
under RLock so callers get immutable snapshots.
9552e80b58