Option to use disk image mtime/ctime for comparing snapshots

[tasket]

A not-so-corner case is where a Wyng user wants to perform frequent backups from very large volumes, even though some of those volumes rarely change. Despite Wyng's existing speed at detecting changes, significant amounts of time can be saved in this use case if Wyng has some way to skip the update_delta_digest() process.

An option like --compare-mtime could be added Since most backup utilities use mtime or ctime, this method could be considered a reliable default for Wyng too, and could shave minutes off monitor and send operations when Petabyte and larger-sized volumes are involved.

It seems this would only work for reflink type storage, not LVM.

[tasket]

To clarify:

During snapshot preparation, the following would be treated as unchanged:

1. deltamap for the volume is empty.
2. and the mtimes and sizes between snap1 and snap2 match completely.

The current code does this check to compare the source volume, but doesn't explicitly compare snap1/snap2.

[tasket]

This has been implemented in prepare_snapshots_reflink() and uses the approach of comparing the timestamp of snap1 with that of the source volume.