Move ChannelManager::default_configuration behind a RwLock

In the next commit we'll add the ability to update the default
configuration used for new channels and which controls some global
decisions in the `ChannelManager`. However, first, we have to put
it behind a mutex so that it can be updated at runtime, which we do
here.

We also take this opportunity to rename `default_configuration` to
simply `config`, since it has been more than only the default
configuration for new channels for some time.

Author: TheBlueMatt

fuzz/src/chanmon_consistency.rs

@@ -771,7 +771,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 			router: &router,
 			message_router: &router,
 			logger,
-			default_config: config,
+			config,
 			channel_monitors: monitor_refs,
 		};