Add db:mv function for copy and move of documents

[zadean]

@zadean If it's not to late for this feature set, I would like to see a db:mv , which like the unix variant could be used for renaming or moving stored items.

I know, this would be possible with, get, delete, put (got resource) cycle, but I think it would be nice convenance function

Originally posted by @grantmacken in https://github.com/zadean/xqerl/pull/25#issuecomment-552578037