Create and update duplicate archives

[tasket]

Add an archive duplication feature to create backups of archives in another location.

Problem

Although commands like rsync -aH --delete may be considered sufficient for making duplicates intact, there are a couple of drawbacks:

1. rsync, cp, etc. aren't aware of the dichotomy between data and metadata in Wyng archives, so any neglect by the user when running these tools (not heeding error conditions) could result in a seemingly intact archive that is corrupt. Although Wyng provides multiple ways to check for errors, relying on verification is not the most preventative process.

2. There's no possibility for selecting which volumes or sessions within an archive to duplicate (using traditional tools). Users may want to prioritize only certain volumes or sessions for their backup-of-a-backup.

Solution

A Wyng duplication function could make and refresh duplicate archives using the same safety patterns (data first, metadata last) employed when creating original archives. It would also be possible to add some level of selectivity (per volume, etc) at some point.

It could be of further help if the duplicate archive were marked as having a special status and possibly with a different uuid; this would be to avoid temptation of backing up to two+ copies absentmindedly with users thinking they are the same archive.

Notes

• Encryption keys could be the same between two archives. If they are not, then duplication would involve a re-encryption process.
• Duplication function would have to acquire two unique coexisting instances of Destination class and probably ArchiveSet as well. Some currently 'independent' helper functions may have to be moved into those classes to raise their effective encapsulation.

Related

#140
#175
#184