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/network/availability-distribution/src/tests/state.rs

@@ -29,7 +29,8 @@ use sc_network as network;
 use sc_network::IfDisconnected;
 use sc_network::config as netconfig;
 
-use polkadot_subsystem::{ActiveLeavesUpdate, FromOverseer, OverseerSignal, ActivatedLeaf,
+use polkadot_subsystem::{
+	ActiveLeavesUpdate, FromOverseer, OverseerSignal, ActivatedLeaf, LeafStatus,
 	messages::{
 		AllMessages, AvailabilityDistributionMessage, AvailabilityStoreMessage, NetworkBridgeMessage,
 		RuntimeApiMessage, RuntimeApiRequest,
@@ -173,6 +174,7 @@ impl TestState {
 				activated: smallvec![ActivatedLeaf {
 					hash: new.clone(),
 					number: 1,
+					status: LeafStatus::Fresh,
 					span: Arc::new(jaeger::Span::Disabled),
 				}],
 				deactivated: smallvec![old.clone()],