DB adapter for Datomic

Sequence is compatible with any database allowing conditional puts. We should implement connectors to multiple databases to allow the usage in different environments. This will be the first adapter implementation. It will pave the way for new adapter to be created. The implementation must include:

• A better decoupling of the DB adapter logic from the ledger
• The implementation of the adapter for the Datomic database
• Proper documentation about how to implement new adapters