Punish offline validators, aura-style (#1216)

* make offline-reporting infrastructure more generic

* add a listener-trait for watching when the timestamp has been set

* prevent inclusion of empty offline reports

* add test for exclusion

* generate aura-offline reports

* ability to slash many times for being offline "multiple" times

* Logic for punishing validators for missing aura steps

* stub tests

* pave way for verification of timestamp vs slot

* alter aura import queue to wait for timestamp

* check timestamp matches seal

* do inherent check properly

* service compiles

* all tests compile

* test srml-aura logic

* aura tests pass

* everything builds

* some more final tweaks to block authorship for aura

* switch to manual delays before step

* restore substrate-consensus-aura to always std and address grumbles

* update some state roots in executor tests

* node-executor tests pass

* get most tests passing

* address grumbles

substrate/srml/consensus/src/mock.rs

@@ -34,7 +34,7 @@ impl Trait for Test {
 	const NOTE_OFFLINE_POSITION: u32 = 1;
 	type Log = DigestItem;
 	type SessionKey = u64;
-	type OnOfflineValidator = ();
+	type InherentOfflineReport = ::InstantFinalityReportVec<()>;
 }
 impl system::Trait for Test {
 	type Origin = Origin;