wallet: Introduce database handle wrapper

Abstract database handle from explicit strFilename into
CWalletDBWrapper.

Also move CWallet::Backup to db.cpp - as it deals with representation
details this is a database specific operation.

src/qt/test/wallettests.cpp

@@ -86,7 +86,8 @@ void WalletTests::walletTests()
     TestChain100Setup test;
     test.CreateAndProcessBlock({}, GetScriptForRawPubKey(test.coinbaseKey.GetPubKey()));
     bitdb.MakeMock();
-    CWallet wallet("wallet_test.dat");
+    std::unique_ptr<CWalletDBWrapper> dbw(new CWalletDBWrapper(&bitdb, "wallet_test.dat"));
+    CWallet wallet(std::move(dbw));
     bool firstRun;
     wallet.LoadWallet(firstRun);
     {