AttemptStart

What RunQueue.recordAttempt did (Stage 5 rule 4).

• Recorded: the attempt's RunId is recorded, the last of entry.attempts; the attempt may now touch the thread, classifying it first (AttemptPlan).
• Exhausted: the entry has made maxAttempts attempts, so none is recorded. The worker settles the thread if the entry's input was admitted, then closes the entry Failed(AttemptsExhausted) - a dead letter.
• ThreadDeleted: the thread was deleted since the entry was submitted; the store closed the entry Cancelled(ThreadDeleted) and recorded nothing, so nothing runs.