Disconect announcement_signatures sending from funding_locked

The spec actually requires we never send `announcement_signatures`
(and, thus, `channel_announcement`s) until after six confirmations.
However, we would happily have sent them prior to that as long as
we exchange `funding_locked` messages with our countarparty. Thanks
to re-broadcasting this issue is largely harmless, however it could
have some negative interactions with less-robust peers. Much more
importantly, this represents an important step towards supporting
0-conf channels, where `funding_locked` messages may be exchanged
before we even have an SCID to construct the messages with.

Because there is no ACK mechanism for `announcement_signatures` we
rely on existing channel updates to stop rebroadcasting them - if
we sent a `commitment_signed` after an `announcement_signatures`
and later receive a `revoke_and_ack`, we know our counterparty also
received our `announcement_signatures`. This may resolve some rare
edge-cases where we send a `funding_locked` which our counterparty
receives, but lose connection before the `announcement_signatures`
(usually the very next message) arrives.

Sadly, because the set of places where an `announcement_signatures`
may now be generated more closely mirrors where `funding_locked`
messages may be generated, but they are now separate, there is a
substantial amount of code motion providing relevant parameters
about current block information and ensuring we can return new
`announcement_signatures` messages.

Author: TheBlueMatt

lightning/src/ln/chanmon_update_fail_tests.rs

@@ -20,6 +20,7 @@ use chain::channelmonitor::ChannelMonitor;
 use chain::transaction::OutPoint;
 use chain::{ChannelMonitorUpdateErr, Listen, Watch};
 use ln::channelmanager::{ChannelManager, ChannelManagerReadArgs, RAACommitmentOrder, PaymentSendFailure};
+use ln::channel::AnnouncementSigsState;
 use ln::features::InitFeatures;
 use ln::msgs;
 use ln::msgs::{ChannelMessageHandler, RoutingMessageHandler};
@@ -1402,6 +1403,11 @@ fn monitor_failed_no_reestablish_response() {
 	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
 	let mut nodes = create_network(2, &node_cfgs, &node_chanmgrs);
 	let channel_id = create_announced_chan_between_nodes(&nodes, 0, 1, InitFeatures::known(), InitFeatures::known()).2;
+	{
+		let mut lock;
+		get_channel_ref!(nodes[0], lock, channel_id).announcement_sigs_state = AnnouncementSigsState::PeerReceived;
+		get_channel_ref!(nodes[1], lock, channel_id).announcement_sigs_state = AnnouncementSigsState::PeerReceived;
+	}
 
 	// Route the payment and deliver the initial commitment_signed (with a monitor update failure
 	// on receipt).