filebackend: improve FileBackendMultiWrite consistencyCheck()/resyncFiles()

Report file stat errors and sha1/stat mismatches in consistencyCheck().
This will trigger resyncFiles() which will make second attempt to check
the consistency while also fixing any problems if possible. Make sure
that it also bails out if such errors occur again.

Improve consistencyCheck()/resyncFiles() variable naming and add more
comments to FileBackend::UNKNOWN.

Also replace wfDebugLog() calls with PSR logger calls and wfTimestamp()
calls with ConvertibleTimestamp::convert() calls.

Bug: T231086
Change-Id: I69bcee636c6d99970e9a6448bb8296c0790c7254