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/session/src/lib.rs

@@ -261,7 +261,7 @@ mod tests {
 		const NOTE_OFFLINE_POSITION: u32 = 1;
 		type Log = DigestItem;
 		type SessionKey = u64;
-		type OnOfflineValidator = ();
+		type InherentOfflineReport = ();
 	}
 	impl system::Trait for Test {
 		type Origin = Origin;
@@ -278,6 +278,7 @@ mod tests {
 	impl timestamp::Trait for Test {
 		const TIMESTAMP_SET_POSITION: u32 = 0;
 		type Moment = u64;
+		type OnTimestampSet = ();
 	}
 	impl Trait for Test {
 		type ConvertAccountIdToSessionKey = Identity;