LevelDB block and coin databases

Split off CBlockTreeDB and CCoinsViewDB into txdb-*.{cpp,h} files,
implemented by either LevelDB or BDB.

Based on code from earlier commits by Mike Hearn in his leveldb
branch.

src/init.cpp

@@ -2,7 +2,7 @@
 // Copyright (c) 2009-2012 The Bitcoin developers
 // Distributed under the MIT/X11 software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
-#include "db.h"
+#include "txdb.h"
 #include "walletdb.h"
 #include "bitcoinrpc.h"
 #include "net.h"