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/test-runtime/src/lib.rs

@@ -67,6 +67,7 @@ pub const VERSION: RuntimeVersion = RuntimeVersion {
 	authoring_version: 1,
 	spec_version: 1,
 	impl_version: 1,
+	apis: apis_vec!([]),
 };
 
 fn version() -> RuntimeVersion {