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/client/src/error.rs

@@ -99,6 +99,18 @@ error_chain! {
 			description("Error decoding call result")
 			display("Error decoding call result of {}", method)
 		}
+
+		/// Last finalized block not parent of current.
+		NonSequentialFinalization(s: String) {
+			description("Did not finalize blocks in sequential order."),
+			display("Did not finalize blocks in sequential order."),
+		}
+
+		/// Safety violation: new best block not descendent of last finalized.
+		NotInFinalizedChain {
+			description("Potential long-range attack: block not in finalized chain."),
+			display("Potential long-range attack: block not in finalized chain."),
+		}
 	}
 }