refactor: Move fs.* to util/fs.*

The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.

src/node/mempool_persist_args.cpp

@@ -4,7 +4,7 @@
 
 #include <node/mempool_persist_args.h>
 
-#include <fs.h>
+#include <util/fs.h>
 #include <util/system.h>
 #include <validation.h>