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/init.cpp

@@ -10,6 +10,7 @@
 #include <init.h>
 
 #include <kernel/checks.h>
+#include <kernel/mempool_persist.h>
 
 #include <addrman.h>
 #include <banman.h>
@@ -104,6 +105,8 @@
 #include <zmq/zmqrpc.h>
 #endif
 
+using kernel::DumpMempool;
+
 using node::CacheSizes;
 using node::CalculateCacheSizes;
 using node::ChainstateLoadVerifyError;