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/service/src/chain_ops.rs

@@ -102,8 +102,9 @@ pub fn import_blocks<F, E, R>(config: FactoryFullConfiguration<F>, exit: E, mut
 		}
 		match SignedBlock::decode(&mut input) {
 			Some(block) => {
+				// TODO: non-instant finality.
 				let header = client.check_justification(block.block.header, block.justification.into())?;
-				client.import_block(BlockOrigin::File, header, Some(block.block.extrinsics))?;
+				client.import_block(BlockOrigin::File, header, Some(block.block.extrinsics), true)?;
 			},
 			None => {
 				warn!("Error reading block data.");