Differentiate between ClosedBlock (can be reopened) and LockedBlock (cannot).

`ClosedBlock`s still keep the pre-finalised state (i.e. state after the last transaction).
`LockedBlock`s do not. New mining algo needs to reopen these `ClosedBlock`s, however enactment
system does not (and `ClosedBlock`s are slower & more hungry), hence the distinction.

ethcore/src/tests/client.rs

@@ -147,5 +147,5 @@ fn can_mine() {
 	let b = client.prepare_sealing(Address::default(), x!(31415926), vec![], vec![]).0.unwrap();
 
 	assert_eq!(*b.block().header().parent_hash(), BlockView::new(&dummy_blocks[0]).header_view().sha3());
-	assert!(client.try_seal(b, vec![]).is_ok());
+	assert!(client.try_seal(b.lock(), vec![]).is_ok());
 }