io: allow freeing of io_conn at any time.

io_close() currently marks the io_conn for freeing, but doesn't
actually do it.  This is a problem for tal() users, because we can't
just call it in the parent's constructor.

Make io_close() just tal_free() + return &io_conn_freed (a magic
io_plan pointer).

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