ccan/io: keep always pointers to plans, not a linked list.

Duplex io_conns can be in the always list twice, and it was a source
of bugs, but I didn't want to add a second list_node.  Since there are
not many always at once, it's better (and more space-efficient) to use
a simple pointer array.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>