Drop the dist HashMap in routing, replacing it with a Vec.

Now that we have unique, dense, 32-bit identifiers for all the
nodes in our network graph, we can store the per-node information
when routing in a simple `Vec` rather than a `HashMap`. This avoids
the overhead of hashing and table scanning entirely, for a nice
"simple" optimization win.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -1067,6 +1067,14 @@ impl<'a> DirectedChannelInfo<'a> {
 	/// Refers to the `node_id` receiving the payment from the previous hop.
 	#[inline]
 	pub fn target(&self) -> &'a NodeId { if self.from_node_one { &self.channel.node_two } else { &self.channel.node_one } }
+
+	/// Returns the source node's counter
+	#[inline]
+	pub(super) fn source_counter(&self) -> u32 { if self.from_node_one { self.channel.node_one_counter } else { self.channel.node_two_counter } }
+
+	/// Returns the target node's counter
+	#[inline]
+	pub(super) fn target_counter(&self) -> u32 { if self.from_node_one { self.channel.node_two_counter } else { self.channel.node_one_counter } }
 }
 
 impl<'a> fmt::Debug for DirectedChannelInfo<'a> {