scripted-diff: Move mempool_args to src/node

It is part of the node library. Also, it won't be moved to the kernel
lib, as it will be pruned of ArgsManager.

-BEGIN VERIFY SCRIPT-
 # Move module
 git mv src/mempool_args.cpp src/node/
 git mv src/mempool_args.h   src/node/
 # Replacements
 sed -i 's:mempool_args\.h:node/mempool_args.h:g'     $(git grep -l mempool_args)
 sed -i 's:mempool_args\.cpp:node/mempool_args.cpp:g' $(git grep -l mempool_args)
 sed -i 's:MEMPOOL_ARGS_H:NODE_MEMPOOL_ARGS_H:g'      $(git grep -l MEMPOOL_ARGS_H)
-END VERIFY SCRIPT-

src/node/mempool_args.h

@@ -0,0 +1,27 @@
+// Copyright (c) 2022 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_NODE_MEMPOOL_ARGS_H
+#define BITCOIN_NODE_MEMPOOL_ARGS_H
+
+#include <optional>
+
+class ArgsManager;
+class CChainParams;
+struct bilingual_str;
+namespace kernel {
+struct MemPoolOptions;
+};
+
+/**
+ * Overlay the options set in \p argsman on top of corresponding members in \p mempool_opts.
+ * Returns an error if one was encountered.
+ *
+ * @param[in]  argsman The ArgsManager in which to check set options.
+ * @param[in,out] mempool_opts The MemPoolOptions to modify according to \p argsman.
+ */
+[[nodiscard]] std::optional<bilingual_str> ApplyArgsManOptions(const ArgsManager& argsman, const CChainParams& chainparams, kernel::MemPoolOptions& mempool_opts);
+
+
+#endif // BITCOIN_NODE_MEMPOOL_ARGS_H