Don't bump the next_node_counter when using a removed counter

If we manage to pull a `node_counter` from `removed_node_counters`
for reuse, `add_channel_between_nodes` would `unwrap_or` with the
`next_node_counter`-incremented value. This visually looks right,
except `unwrap_or` is always called, causing us to always increment
`next_node_counter` even if we don't use it.

This will result in the `node_counter`s always growing any time we
add a new node to our graph, leading to somewhat larger memory
usage when routing and a debug assertion failure in
`test_node_counter_consistency`.

The fix is trivial, this is what `unwrap_or_else` is for.

Author: TheBlueMatt

lightning/src/routing/gossip.rs

@@ -1898,7 +1898,9 @@ impl<L: Deref> NetworkGraph<L> where L::Target: Logger {
 				IndexedMapEntry::Vacant(node_entry) => {
 					let mut removed_node_counters = self.removed_node_counters.lock().unwrap();
 					**chan_info_node_counter = removed_node_counters.pop()
-						.unwrap_or(self.next_node_counter.fetch_add(1, Ordering::Relaxed) as u32);
+						.unwrap_or_else(|| {
+							self.next_node_counter.fetch_add(1, Ordering::Relaxed) as u32
+						});
 					node_entry.insert(NodeInfo {
 						channels: vec!(short_channel_id),
 						announcement_info: None,