Use block timestamps as the min for generated update messages.

Fixes issue #493 and should resolve some issues where other nodes
(incorrectly) reject channel_update/node_announcement messages
which have a serial number that is not a relatively recent
timestamp.

lightning/src/ln/channel.rs

@@ -3175,6 +3175,7 @@ impl<ChanSigner: ChannelKeys> Channel<ChanSigner> {
 		}
 		if header.bitcoin_hash() != self.last_block_connected {
 			self.last_block_connected = header.bitcoin_hash();
+			self.channel_update_count = cmp::max(self.channel_update_count, header.time);
 			if let Some(channel_monitor) = self.channel_monitor.as_mut() {
 				channel_monitor.last_block_hash = self.last_block_connected;
 			}

lightning/src/ln/channelmanager.rs

@@ -2759,6 +2759,19 @@ impl<ChanSigner: ChannelKeys, M: Deref + Sync + Send, T: Deref + Sync + Send, K:
 		}
 		self.latest_block_height.store(height as usize, Ordering::Release);
 		*self.last_block_hash.try_lock().expect("block_(dis)connected must not be called in parallel") = header_hash;
+		loop {
+			// Update last_node_announcement_serial to be the max of its current value and the
+			// block timestamp. This should keep us close to the current time without relying on
+			// having an explicit local time source.
+			// Just in case we end up in a race, we loop until we either successfully update
+			// last_node_announcement_serial or decide we don't need to.
+			let old_serial = self.last_node_announcement_serial.load(Ordering::Acquire);
+			if old_serial < header.time as usize {
+				if self.last_node_announcement_serial.compare_exchange(old_serial, header.time as usize, Ordering::AcqRel, Ordering::Relaxed).is_ok() {
+					break;
+				}
+			} else { break; }
+		}
 	}
 
 	/// We force-close the channel without letting our counterparty participate in the shutdown