Remove redundant task queueing to start transaction #487

https://w3c.github.io/IndexedDB/#transaction-start

When an implementation is able to enforce the constraints for the transaction’s (...) the implementation must queue a database task to start the transaction asynchronously

Starting a transaction doesn't seem to require queuing a task, because the state change only affect parallel steps (of https://w3c.github.io/IndexedDB/#async-execute-request).

The event-loop will notice when a transaction has started only when the event are fired from within the task queued when an request has been executed.

You could also add a "Wait for transaction to have started" step to the parallel part of #async-execute-request.