BerkeleyRODatabase is intended for use after BDB is removed, so it needs
to be able to read all of the records from a BDB file. Thus an
independent deserializer for BDB data files is implemented in it. This
deserializer is targeted towards the data files that Bitcoin Core
creates so it does not fully support all of BDB's features (e.g. other
database types, encryption, etc.).
Implement ReadKey and HasKey of BerkeleyROBatch, and Next of BerkeleyROCursor.
Also adds the containers for records to BerkeleyRODatabase so that
BerkeleyROBatch will be able to access the records.