We have a lot of infrastructure to delay local channel_updates to
avoid spamming on each peer reconnect; we had to keep tracking of
pending ones though, in case we needed the very latest for sending an
error when failing an HTLC.
Instead, it's far simpler to set the local_disabled flag on a channel
when we disconnect, but only send a disabling channel_update if we
actually fail an HTLC.
Note: handle_channel_update() TAKES update (due to tal_arr_dup), but we
didn't use that before. Now we do, add annotation.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>