Logger for long-hold warnings.
Run work under the lock of key.
Document key (fileLockKey or a source reference).
Operation name (warnings, timeout error).
The work (must not take the same key again).
Bounded wait.
What work returns.
Run work holding the locks of every key, taken one at a time in sorted
order (duplicates once).
Document keys.
Operation name.
The work.
Bounded wait (per lock).
What work returns.
The per-document locks of one processor.