Make metadata handling more robust

Metadata handling should be more robust, in particular the importer should:

• Handle metadata from very old DokuWiki versions that didn't have the separation into current/persistent.
• Handle values only being present in either current or persistent metadata. From looking at various metadata files, metadata can be missing in either of those as old versions seem to have incomplete current metadata while more recent versions seem to lack metadata in the persistent metadata when pages were externally (i.e., not through DokuWiki) created.
• Ignore unreadable metadata (unserialize errors).