adds plugin CopyArchiveTags

[IceBreeze]

I took advantage of the issue #966 to try some suggestions like:

• exception handling for the plugins
• named parameters

They are implemented inside the plugin because I think they are too invasive to be put elsewhere at the moment.

Also I added a little helper method to retrieve the tags from an archive and I slightly reworked various get_archive* methods internally, but I couldn't come up with a better name for h_get_archive for the one that needs the Redis instance to be passed.

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