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/init.cpp

@@ -3,6 +3,7 @@
 // Distributed under the MIT/X11 software license, see the accompanying
 // file license.txt or http://www.opensource.org/licenses/mit-license.php.
 #include "db.h"
+#include "walletdb.h"
 #include "bitcoinrpc.h"
 #include "net.h"
 #include "init.h"