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

contrib/devtools/iwyu/bitcoin.core.imp

@@ -3,4 +3,5 @@
   { include: [ "<bits/termios-c_lflag.h>", private, "<termios.h>", public ] },
   { include: [ "<bits/termios-struct.h>", private, "<termios.h>", public ] },
   { include: [ "<bits/termios-tcflow.h>", private, "<termios.h>", public ] },
+  { include: [ "<bits/chrono.h>", private, "<chrono>", public ] },
 ]