Instead of returning a Dbc (BDB cursor object) and having the caller
deal with the cursor, make BerkeleyBatch handle the cursor internally.
This prepares BerkeleyBatch to work with other database systems as Dbc
objects are BDB specific.
In order to override these later, the specific details of how the Read,
Write, Erase, and Exists functions interact with the actual database
file need to go into functions that are not templated.
