Audit each individual thread pool size

[steviez]

Problem

The solana-validator process creates a lot of threads, including a number of threadpools. A staked node running v1.17 was observed to have more than 800 threads. The exact number may vary a little depending on some configuration as well as the number of physical cores the machine has (several pools scale with the number of cores). This is a lot of threads, and all of these threads add non-trivial overhead to the system.

Proposed Solution

Examine each thread pool more closely, and reduce thread pool sizes where appropriate. Even if a particular workflow actually utilizes a very high level of parallelism, it may be the case that we should still put a hard upper bound in place instead of letting the thread pool size scale with the number of cores.