The rdb module was not properly designed, which is fixed in this change. The module had three classes:
1) The abstract class `RelationalDB`.
2) The abstract class `SQLiteDatabase`, which inherited from `RelationalDB` and added some pure virtual methods.
3) The concrete class `SQLiteDatabaseImp`, which inherited from `SQLiteDatabase` and implemented all methods.
The updated code simplifies this as follows:
* The `SQLiteDatabaseImp` has become `SQLiteDatabase`, and
* The former `SQLiteDatabase `has merged with `RelationalDatabase`.
This change modularizes the `WalletDB` and `Manifest`. Note that the wallet db has nothing to do with account wallets and it stores node configuration, which is why it depends on the manifest code.
