Allow configuration of all validator thread-pool sizes via CLI

[steviez]

Problem

As mentioned in #35, agave-validator (previously solana-validator) spawns lots of thread pools / tokio runtimes. For the majority of these, the size of the pool/runtime is specified by a constant in code, or configured to scale with the number of cores on the machine.

Some of sizes were carefully chosen and may still be an appropriate amount. Other values might have been unintentionally set to scale with number of cores (this is the default behavior when the number of threads is not specifically set). Or, some values might have become "incorrect" as the workload of the cluster changes / refactors occur and shift where heavier operations occur.

In any case, being able to control these sizes could be a powerful knob to turn.

Proposed Solution

• Add CLI flags + plumbing for thread pool size parameters
• The addition of the CLI flags should NOT alter any thread pool sizes
  • This can be tackled in subsequent work tracked by Audit each individual thread pool size #35
• The flags should probably be hidden, maybe forever. Or, if we make them public, they'll need a warning along the lines of "you should only be using this if you know what you're doing"
• Finally, having the ability to configure all sizes should make the rayon-threadlimit crate obsolete, altho we may need to follow a proper deprecation sequence for that crate