Replace Arc.clone()s with Arc::clone() in background-processor

`arc.clone()` leaves the code somewhat ambiguous to whether we're
doing an expensive deep-clone operation or if we're doing a
(relatively) cheap pointer-copy-and-atomic-increment operation.

More importantly, it leaves entirely unclear what the semantics of
the object we just created are - does updating it update the
original or leave it be?

Thus, here, we replace `arc.clone()` calls with `Arc::clone()` in
`lightning-background-processor/src/lib.rs`

Author: TheBlueMatt

lightning-background-processor/src/lib.rs

@@ -1021,7 +1021,7 @@ impl BackgroundProcessor {
 		K::Target: 'static + KVStore,
 	{
 		let stop_thread = Arc::new(AtomicBool::new(false));
-		let stop_thread_clone = stop_thread.clone();
+		let stop_thread_clone = Arc::clone(&stop_thread);
 		let handle = thread::spawn(move || -> Result<(), std::io::Error> {
 			let event_handler = |event| {
 				let network_graph = gossip_sync.network_graph();