Database binary stores should not hold on to PreparedStatements

Our current implementation for the DatabaseBinaryStore will create up-front and hold on to PreparedStatement objects for each of the operations performed against a database (e.g. retrieving a binary, marking it as unused etc).
The problem with this approach is that depending on the driver implementation, each PreparedStatement can hod on to, in turn, a Connection instance. Depending on the application server/datasource configuration this can be closed/expired preventing the binary store from functioning correctly.