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/txdb.h

@@ -0,0 +1,14 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// 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.
+#ifndef BITCOIN_TXDB_H
+#define BITCOIN_TXDB_H
+
+#ifdef USE_LEVELDB
+#include "txdb-leveldb.h"
+#else
+#include "txdb-bdb.h"
+#endif
+
+#endif // BITCOIN_TXDB_H