Finality notification streams (#791)

* 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`

* move `tree_route` to client common

* tree_route tests

* return slices in TreeRoute

* apply finality up to parent

* add `finalize_block` call

* adjust formatting

* finality notifications and add last finalized block to chain info

* exhaustive match and comments

* fix sync tests by using non-instant finality

substrate/core/network/src/service.rs

@@ -165,7 +165,8 @@ impl<B: BlockT + 'static, S: Specialization<B>, H: ExHashT> Service<B, S, H> {
 	/// Creates and register protocol with the network service
 	pub fn new(params: Params<B, S, H>, protocol_id: ProtocolId) -> Result<Arc<Service<B, S, H>>, Error> {
 		let chain = params.chain.clone();
-		let import_queue = Arc::new(AsyncImportQueue::new());
+		// TODO: non-insatnt finality.
+		let import_queue = Arc::new(AsyncImportQueue::new(true));
 		let handler = Arc::new(ProtocolHandler {
 			protocol: Protocol::new(
 				params.config,