walletdb: Introduce WalletDatabase abstract class

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

src/wallet/walletdb.cpp

@@ -1012,20 +1012,20 @@ bool IsWalletLoaded(const fs::path& wallet_path)
 }
 
 /** Return object for accessing database at specified path. */
-std::unique_ptr<BerkeleyDatabase> CreateWalletDatabase(const fs::path& path)
+std::unique_ptr<WalletDatabase> CreateWalletDatabase(const fs::path& path)
 {
     std::string filename;
     return MakeUnique<BerkeleyDatabase>(GetWalletEnv(path, filename), std::move(filename));
 }
 
 /** Return object for accessing dummy database with no read/write capabilities. */
-std::unique_ptr<BerkeleyDatabase> CreateDummyWalletDatabase()
+std::unique_ptr<WalletDatabase> CreateDummyWalletDatabase()
 {
     return MakeUnique<BerkeleyDatabase>();
 }
 
 /** Return object for accessing temporary in-memory database. */
-std::unique_ptr<BerkeleyDatabase> CreateMockWalletDatabase()
+std::unique_ptr<WalletDatabase> CreateMockWalletDatabase()
 {
     return MakeUnique<BerkeleyDatabase>(std::make_shared<BerkeleyEnvironment>(), "");
 }