NetworkGraph: One pre-allocate memory on mainnet

Previously, we'd always pre-allocate memory for the node and channel
maps based on mainnet numbers, even if we're on another network like
`Regest`. Here, we only apply the estimates if we're actually on
`Network::Bitcoin`, which should reduce the `NetworkGraph`'s memory
footprint considerably in tests.

Author: tnull

lightning/src/routing/gossip.rs

@@ -1802,12 +1802,18 @@ where
 {
 	/// Creates a new, empty, network graph.
 	pub fn new(network: Network, logger: L) -> NetworkGraph<L> {
+		let (node_map_cap, chan_map_cap) = if matches!(network, Network::Bitcoin) {
+			(NODE_COUNT_ESTIMATE, CHAN_COUNT_ESTIMATE)
+		} else {
+			(0, 0)
+		};
+
 		Self {
 			secp_ctx: Secp256k1::verification_only(),
 			chain_hash: ChainHash::using_genesis_block(network),
 			logger,
-			channels: RwLock::new(IndexedMap::with_capacity(CHAN_COUNT_ESTIMATE)),
-			nodes: RwLock::new(IndexedMap::with_capacity(NODE_COUNT_ESTIMATE)),
+			channels: RwLock::new(IndexedMap::with_capacity(chan_map_cap)),
+			nodes: RwLock::new(IndexedMap::with_capacity(node_map_cap)),
 			next_node_counter: AtomicUsize::new(0),
 			removed_node_counters: Mutex::new(Vec::new()),
 			last_rapid_gossip_sync_timestamp: Mutex::new(None),