WAL overwrites across clusters with same name

The in-tree barman archiver prevents you from archiving wal files if the object stores already has wals in there generated by a different cluster with the same name.

The plugin instead allows doesn't have this protection and allows to overwrite the files, so there could be wal files from different sources.

Start by comparing the plugin's archiving behavior with the in-tree barman archiver, then reproduce the case with existing WALs from another cluster sharing the same name. The change is complete when the plugin refuses to overwrite WAL files from a different source, with tests covering that protection.

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