walletdb: Introduce WalletDatabase abstract class

Make WalletDatabase actually an abstract class and not just a typedef
for BerkeleyDatabase. Have BerkeleyDatabase inherit this class.

src/qt/rpcconsole.cpp

@@ -24,7 +24,6 @@
 #include <univalue.h>
 
 #ifdef ENABLE_WALLET
-#include <wallet/bdb.h>
 #include <wallet/db.h>
 #include <wallet/wallet.h>
 #endif