Move CWalletDB code to new walletdb module.

In addition to standard code separation, this change opens the door
to fixing several include inter-dependencies.

src/qt/optionsmodel.cpp

@@ -3,7 +3,7 @@
 #include <QSettings>
 
 #include "init.h"
-#include "db.h"
+#include "walletdb.h"
 
 OptionsModel::OptionsModel(QObject *parent) :
     QAbstractListModel(parent)

src/qt/walletmodel.cpp

@@ -5,7 +5,7 @@
 #include "transactiontablemodel.h"
 
 #include "wallet.h"
-#include "db.h" // for BackupWallet
+#include "walletdb.h" // for BackupWallet
 
 #include <QSet>