Reduce RwLock usage in public interface of NetworkGraph

This isn't a big difference in the API, but it avoids needing to
wrap a given NetworkGraph in a RwLock before passing it, which
makes it much easier to generate C bindings for.

Author: TheBlueMatt

lightning/src/ln/functional_test_utils.rs

@@ -31,7 +31,7 @@ use bitcoin::secp256k1::key::PublicKey;
 
 use std::cell::RefCell;
 use std::rc::Rc;
-use std::sync::{Mutex, RwLock};
+use std::sync::Mutex;
 use std::mem;
 use std::collections::HashMap;
 
@@ -103,7 +103,7 @@ impl<'a, 'b, 'c> Drop for Node<'a, 'b, 'c> {
 				let network_graph_deser = <NetworkGraph>::read(&mut ::std::io::Cursor::new(&w.0)).unwrap();
 				assert!(network_graph_deser == *self.net_graph_msg_handler.network_graph.read().unwrap());
 				let net_graph_msg_handler = NetGraphMsgHandler::from_net_graph(
-					self.chain_monitor, self.logger, RwLock::new(network_graph_deser)
+					self.chain_monitor, self.logger, network_graph_deser
 				);
 				let mut chan_progress = 0;
 				loop {

lightning/src/routing/network_graph.rs

@@ -59,10 +59,10 @@ impl<C: Deref, L: Deref> NetGraphMsgHandler<C, L> where C::Target: ChainWatchInt
 
 	/// Creates a new tracker of the actual state of the network of channels and nodes,
 	/// assuming an existing Network Graph.
-	pub fn from_net_graph(chain_monitor: C, logger: L, network_graph: RwLock<NetworkGraph>) -> Self {
+	pub fn from_net_graph(chain_monitor: C, logger: L, network_graph: NetworkGraph) -> Self {
 		NetGraphMsgHandler {
 			secp_ctx: Secp256k1::verification_only(),
-			network_graph,
+			network_graph: RwLock::new(network_graph),
 			full_syncs_requested: AtomicUsize::new(0),
 			chain_monitor,
 			logger,