Replace includes of boost/filesystem.h with fs.h

This is step one in abstracting the use of boost::filesystem.

src/wallet/walletdb.cpp

@@ -7,6 +7,7 @@
 
 #include "base58.h"
 #include "consensus/validation.h"
+#include "fs.h"
 #include "validation.h" // For CheckTransaction
 #include "protocol.h"
 #include "serialize.h"
@@ -18,7 +19,6 @@
 #include <atomic>
 
 #include <boost/version.hpp>
-#include <boost/filesystem.hpp>
 #include <boost/foreach.hpp>
 #include <boost/thread.hpp>