Don't apply gossip backpressure to non-channel-announcing peers

Don't apply gossip backpressure to non-channel-announcing peers

When we apply the new gossip-async-check backpressure on peer
connections, if a peer has never sent us a `channel_announcement`
at all, we really shouldn't delay reading their messages.

This does so by tracking, on a per-peer basis, whether they've sent
us a channel_announcement, and resetting that state whenever we're
not backlogged.