Introduce notion of finality to substrate (#760)

* finalization for in_mem

* fetch last finalized block

* pruning: use canonical term instead of final

* finalize blocks in full node

* begin to port light client DB

* add tree-route

* keep number index consistent in full nodes

* fix tests

* disable cache and finish porting light client

* add AsMut to system module

* final leaf is always best

* fix all tests

* Fix comment and trace

* removed unused Into call

* add comment on behavior of `finalize_block`

substrate/core/client/src/light/fetcher.rs

@@ -230,7 +230,13 @@ pub mod tests {
 
 		// check remote read proof locally
 		let local_storage = InMemoryBlockchain::<Block>::new();
-		local_storage.insert(remote_block_hash, remote_block_header.clone(), None, None, true);
+		local_storage.insert(
+			remote_block_hash,
+			remote_block_header.clone(),
+			None,
+			None,
+			::backend::NewBlockState::Final,
+		);
 		let local_executor = test_client::LocalExecutor::new();
 		let local_checker = LightDataChecker::new(local_executor);
 		(local_checker, remote_block_header, remote_read_proof, authorities_len)