We assume if they set this to 0 (which nobody did previously), they're
using it as a modern flag and use it to indicate when they're
finished. Otherwise, we count how many blocks they've sent and use
that to determine whether they've finished.
See: https://github.com/lightningnetwork/lightning-rfc/pull/826
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Changed: Protocol: we use `sync_complete` for gossip range query replies, with detection for older spec nodes.