DumpMempool: Pass in dump_path, stop using gArgs

Also introduce node::{ShouldPersistMempool,MempoolPath} helper functions
in node/mempool_persist_args.{h,cpp} which are used by non-kernel
DumpMempool callers to determine whether or not to automatically dump
the mempool and where to dump it to.

src/node/mempool_persist_args.cpp

@@ -0,0 +1,23 @@
+// 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.
+
+#include <node/mempool_persist_args.h>
+
+#include <fs.h>
+#include <util/system.h>
+#include <validation.h>
+
+namespace node {
+
+bool ShouldPersistMempool(const ArgsManager& argsman)
+{
+    return argsman.GetBoolArg("-persistmempool", DEFAULT_PERSIST_MEMPOOL);
+}
+
+fs::path MempoolPath(const ArgsManager& argsman)
+{
+    return argsman.GetDataDirNet() / "mempool.dat";
+}
+
+} // namespace node