Reversion Safety tools for overseer and subsystems (#3104)

* guide: reversion safety

* guide: manage reversion safety in subsystems

* add leaf status to ActivatedLeaf

* add an LRU-cache to overseer for staleness detection

* update ActivatedLeaf usages in tests to contain status field

* add variant where missed accidentally

* add some helpers to LeafStatus

* address grumbles

polkadot/node/subsystem-util/src/lib.rs

@@ -868,7 +868,7 @@ mod tests {
 	use polkadot_node_jaeger as jaeger;
 	use polkadot_node_subsystem::{
 		messages::{AllMessages, CandidateSelectionMessage}, ActiveLeavesUpdate, FromOverseer, OverseerSignal,
-		SpawnedSubsystem, ActivatedLeaf,
+		SpawnedSubsystem, ActivatedLeaf, LeafStatus,
 	};
 	use assert_matches::assert_matches;
 	use futures::{channel::mpsc, executor, StreamExt, future, Future, FutureExt, SinkExt};
@@ -998,6 +998,7 @@ mod tests {
 					ActiveLeavesUpdate::start_work(ActivatedLeaf {
 						hash: relay_parent,
 						number: 1,
+						status: LeafStatus::Fresh,
 						span: Arc::new(jaeger::Span::Disabled),
 					}),
 				)))
@@ -1028,6 +1029,7 @@ mod tests {
 					ActiveLeavesUpdate::start_work(ActivatedLeaf {
 						hash: relay_parent,
 						number: 1,
+						status: LeafStatus::Fresh,
 						span: Arc::new(jaeger::Span::Disabled),
 					}),
 				)))