Move {Load,Dump}Mempool to kernel namespace

Also:
1. Add the newly introduced kernel/mempool_persist.cpp to IWYU CI script
2. Add chrono mapping for iwyu

src/test/fuzz/validation_load_mempool.cpp

@@ -2,6 +2,8 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <kernel/mempool_persist.h>
+
 #include <chainparamsbase.h>
 #include <mempool_args.h>
 #include <node/mempool_persist_args.h>
@@ -17,6 +19,8 @@
 #include <cstdint>
 #include <vector>
 
+using kernel::DumpMempool;
+
 using node::MempoolPath;
 
 namespace {