ScanNow() causes the file watch loop to run once, and ensures
all events have been posted before returning. It allows the
caller to "synchronize" with the pending watcher events, so
is useful for batch updating after it is known file events
have probably occurred.

On some file systems, multiple changes to the same file within
the file system's mod time resolution would be missed. This
issue can be ameliorated by triggering `Write` events if a
size change is detected, even if no corresponding mod time
change occurs.

Changes to the file content without a change to the size or
mod time will still go unnoticed.