Cache: improvements to previous commit
Cache: improvements to previous commit

 * put `recent_problematic_timestamp` onto `Cache` so that it can be
   shared by multiple Manifest instances.
 * make `isProblematicTimestamp` return true on any filesystem error.
 * save 1 syscall by using truncate=true in createFile instead of
   calling `setEndPos`.

andrewrk/zig,andrewrk/zig