vtgate v3: prep work for new router
vtgate v3: removed plan building from V2 API

We currently don't plan to test V3 with V2. So, there's no
need to build V3 plans for the V2 API.
vtgate v3: Simplified async logic in ScatterConn

Rollback and close used to happen asynchronously. This creates
confusion, makes the code less readable and less testable.
And it doesn't buy you much.