move wallet code to separate file

This introduces two new source files, keystore.cpp and wallet.cpp with
corresponding headers. Code is moved from main and db, in a preparation
for a follow-up commit which introduces the classes CWallet and CKeyStore.

src/headers.h

@@ -91,10 +91,8 @@
 #include "serialize.h"
 #include "uint256.h"
 #include "util.h"
-#include "key.h"
 #include "bignum.h"
 #include "base58.h"
-#include "script.h"
 #include "main.h"
 #ifdef GUI
 #include "uibase.h"