Add block number to activated leaves and associated fixes (#2718)

* add number to `ActivatedLeavesUpdate`

* update subsystem util and overseer

* use new ActivatedLeaf everywhere

* sort view

* sorted and limited view in network bridge

* use live block hash only if it's newer

* grumples

polkadot/roadmap/implementers-guide/src/types/overseer-protocol.md

@@ -30,7 +30,7 @@ Indicates a change in active leaves. Activated leaves should have jobs, whereas
 
 ```rust
 struct ActiveLeavesUpdate {
-    activated: [Hash], // in practice, these should probably be a SmallVec
+    activated: [(Hash, Number)], // in practice, these should probably be a SmallVec
     deactivated: [Hash],
 }
 ```