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/core/av-store/src/tests.rs

@@ -33,6 +33,7 @@ use polkadot_node_primitives::{AvailableData, BlockData, PoV};
 use polkadot_node_subsystem_util::TimeoutExt;
 use polkadot_subsystem::{
 	ActiveLeavesUpdate, errors::RuntimeApiError, jaeger, messages::AllMessages, ActivatedLeaf,
+	LeafStatus,
 };
 use polkadot_node_subsystem_test_helpers as test_helpers;
 use sp_keyring::Sr25519Keyring;
@@ -258,6 +259,7 @@ fn runtime_api_error_does_not_stop_the_subsystem() {
 				activated: vec![ActivatedLeaf {
 					hash: new_leaf,
 					number: 1,
+					status: LeafStatus::Fresh,
 					span: Arc::new(jaeger::Span::Disabled),
 				}].into(),
 				deactivated: vec![].into(),
@@ -994,6 +996,7 @@ async fn import_leaf(
 			activated: vec![ActivatedLeaf {
 				hash: new_leaf,
 				number: 1,
+				status: LeafStatus::Fresh,
 				span: Arc::new(jaeger::Span::Disabled),
 			}].into(),
 			deactivated: vec![].into(),