Do not wait in PersistenceNotifier when the persist flag is set

When we had a event which caused us to set the persist flag in a
PersistenceNotifier in between wait calls, we will still wait,
potentially not persisting a ChannelManager when we should.

Worse, for wait_timeout, this caused us to always wait up to the
timeout, but then always return true that a persistence is needed.

Instead, we simply check the persist flag before waiting, returning
immediately if it is set.

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -4084,6 +4084,10 @@ impl PersistenceNotifier {
 		loop {
 			let &(ref mtx, ref cvar) = &self.persistence_lock;
 			let mut guard = mtx.lock().unwrap();
+			if *guard {
+				*guard = false;
+				return;
+			}
 			guard = cvar.wait(guard).unwrap();
 			let result = *guard;
 			if result {
@@ -4099,6 +4103,10 @@ impl PersistenceNotifier {
 		loop {
 			let &(ref mtx, ref cvar) = &self.persistence_lock;
 			let mut guard = mtx.lock().unwrap();
+			if *guard {
+				*guard = false;
+				return true;
+			}
 			guard = cvar.wait_timeout(guard, max_wait).unwrap().0;
 			// Due to spurious wakeups that can happen on `wait_timeout`, here we need to check if the
 			// desired wait time has actually passed, and if not then restart the loop with a reduced wait