Log changes between vdb_metadata.pickle updates

This adds add support to generate a vdb_metadata_delta.json file
which tracks package merges / unmerges that occur between updates to
vdb_metadata.pickle. IndexedVardb can use the delta together with
vdb_metadata.pickle to reconstruct a complete view of /var/db/pkg,
so that it can avoid expensive listdir calls in /var/db/pkg/*.
Note that vdb_metadata.pickle is only updated periodically, in
order to avoid excessive re-writes of a large file.