afh: Make ->chunks_total and ->seconds_total fixed-size.

These members of struct afh_info are stored as 4-byte quantities in
the serialized afhi blob created by save_afhi(), so the structure
should declare them as uint32_t rather than unsigned long.

Fortunately, this bug is benign since save_afhi() uses the write_u32()
helper from portable_io.h which does the right thing, regardless of
the type of the variable passed.