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-