Store additional metadata for each graph DB replica

Problem

Because indexing and replication will take some time, and also indexing can be done by different version of indexer, there could be situations like this (or combination of them):

• there are multiple replicas for a repository but for different version of repo (different head SHA)
• there are multiple replicas for a repository but with a different version of indexer
• a new indexer version is released and we want to re-index repositories which were indexed before with some older version

When querying a knowledge graph, ideally the newest version of graph DB should be used

Proposal

Store additional metadata which will help us to:

• for each repository decide if it has all replicas up-to-date or if re-indexing/replication is needed. We will need to store: repository's last commit SHA, indexing time, version of indexer
• find newest replica for each repository