Restrict ChannelManager persist in fuzzing to when we're told to

In the `chanmon_consistency` fuzz, we currently "persist" the
`ChannelManager` on each loop iteration. With the new logic in the
past few commits to reduce the frequency of `ChannelManager`
persistences, this behavior now leaves a gap in our test coverage -
missing persistence notifications.

In order to cath (common-case) persistence misses, we update the
`chanmon_consistency` fuzzer to no longer persist the
`ChannelManager` unless the waker was woken and signaled to
persist, possibly reloading with a previous `ChannelManager` if we
were not signaled.

Author: TheBlueMatt

fuzz/src/chanmon_consistency.rs

@@ -66,6 +66,7 @@ use std::cmp::{self, Ordering};
 use hashbrown::{HashSet, hash_map, HashMap};
 use std::sync::{Arc,Mutex};
 use std::sync::atomic;
+use std::time::Duration;
 use std::io::Cursor;
 use bitcoin::bech32::u5;
 
@@ -1248,12 +1249,19 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out) {
 			_ => test_return!(),
 		}
 
-		node_a_ser.0.clear();
-		nodes[0].write(&mut node_a_ser).unwrap();
-		node_b_ser.0.clear();
-		nodes[1].write(&mut node_b_ser).unwrap();
-		node_c_ser.0.clear();
-		nodes[2].write(&mut node_c_ser).unwrap();
+		const NO_TIME: Duration = Duration::from_secs(0);
+		if nodes[0].get_event_or_persistence_needed_future().wait_timeout(NO_TIME) == Ok(true) {
+			node_a_ser.0.clear();
+			nodes[0].write(&mut node_a_ser).unwrap();
+		}
+		if nodes[1].get_event_or_persistence_needed_future().wait_timeout(NO_TIME) == Ok(true) {
+			node_b_ser.0.clear();
+			nodes[1].write(&mut node_b_ser).unwrap();
+		}
+		if nodes[2].get_event_or_persistence_needed_future().wait_timeout(NO_TIME) == Ok(true) {
+			node_c_ser.0.clear();
+			nodes[2].write(&mut node_c_ser).unwrap();
+		}
 	}
 }