Remove unbounded queues from thread pool executors

[jasontedor]

Today, the scaling thread pool executors have unbounded queues by default, and the fixed thread pool executors can have unbounded queues if their size is set to a negative value. Unbounded queues are dangerous and should be eliminated. I think we should remove the ability for any of the thread pool executors to have unbounded queues but maybe (some of?) the scaling thread pool executors should get a caller-runs rejection policy?

Relates #14448

[jasontedor]

We discussed this in Fix-it-Friday and arrived at the following proposal:

• remove custom thread pools Remove custom thread pools #17915
• register proper settings for thread pools
• remove unbounded queues Remove unbounded queues from thread pool executors #18491
• place a hard bound of 256 * max number of threads in pool for a bound on the queue Limit the maximum queue_size for fixed thread pools? #14448

[jasontedor]

Closed in favor of #18613