Reuse thread state when prewarming addresses in parallel

[benaadams]

Changes

• When prewarming addresses no state is changed, so the state can be reused for all prewarming that happens on the parallel thread and doesn't need to go via reset and pool.
• Also plumb through CancellationToken to complete early if block finishes before prewarming.

Types of changes

What types of changes does your code introduce?

• Optimization

Testing

Requires testing

• No