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

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

src/wallet/db.cpp

@@ -6,6 +6,7 @@
 #include "db.h"
 
 #include "addrman.h"
+#include "fs.h"
 #include "hash.h"
 #include "protocol.h"
 #include "util.h"
@@ -17,7 +18,6 @@
 #include <sys/stat.h>
 #endif
 
-#include <boost/filesystem.hpp>
 #include <boost/foreach.hpp>
 #include <boost/thread.hpp>
 #include <boost/version.hpp>