aura: finalize blocks (#9692)

* aura: emit ancestry actions for finalizing blocks

* aura: refactor is_epoch_end to get finalized blocks as argument

* ethcore: add is_epoch_end_light method to Engine

The full client now tracks finality by querying the engine on each block import,
and it also persists the finalization state to the DB. For the light client
current it doesn't persist finality information and only keeps track of finality
for epoch signals, by calling `is_epoch_end_light`. This method implements the
previously existing logic of building finality for all the blocks in the current
epoch and then checking the finalized blocks against the transition store.

* ethcore: allow finalizing current block

* aura: fix construction of finality proof

* aura: fix warnings

- missing docs for is_epoch_end_light
- unused method unfinalized_hashes in RollingFinality

* aura: fix clone on copy types

ethcore/light/src/client/mod.rs

@@ -313,7 +313,7 @@ impl<T: ChainDataFetcher> Client<T> {
 					The node may not be able to synchronize further.", e);
 			}
 
-			let epoch_proof = self.engine.is_epoch_end(
+			let epoch_proof = self.engine.is_epoch_end_light(
 				&verified_header,
 				&|h| self.chain.block_header(BlockId::Hash(h)).and_then(|hdr| hdr.decode().ok()),
 				&|h| self.chain.pending_transition(h),