Use mongo state to update timestamp

[kellrott]

The timestamp method should show when underlying database has changed. Currently mongo driver only watches changes that it makes, but if two arachne servers point at the same mongo db, they won't recognize when the db has changed. Need to use something like https://docs.mongodb.com/manual/changeStreams/ to update timestamp

[adamstruck]

This same problem also exists for elasticsearch

[kellrott]

Will https://stackoverflow.com/questions/27255596/elasticsearch-index-last-update-time work for elasticsearch ?