Generalised Transaction Queue API (#741)

* First effort

* API versioning

* Introduce validate_transaction

* Introduce the API plus fixes.

* Docs

* Typo

* Add longevity parameter to transaction validity info.

substrate/core/rpc/src/chain/tests.rs

@@ -202,6 +202,7 @@ fn should_return_runtime_version() {
 			authoring_version: 1,
 			spec_version: 1,
 			impl_version: 1,
+			apis: (&[][..]).into()
 		}
 	);
 }