Add CLI args for AccountsDb thread pools to validator

[steviez]

Done: This PR is on top of #3270 and should ship after it.
Done: Rebase on top of #3233 after it ships

Problem

See #105 - we want a way to control the size of all thread pools

Summary of Changes

• Add args for the following thread pools:
  • AccountsDb clean
  • AccountsDb "foreground"
  • Accounts Index

[steviez]

I have other commits to add the other two threadpools (hash and "tx processing"), but left them off to simplify the diff while we figure a route forward. I do want to get the account index num threads hooked up as well

[brooksprumo]

Code looks good! Just a rename request:

[steviez]

Just did a series of rebases to clean up the commit history; happy with how minimal these commits are after the handful of cleanup PR's that came before them

I think the logic is straight forward, but I would appreciate thoughts on the "process" pool naming. This is the one that I think I've heard y'all call "foreground" before. I wasn't sure if we wanted to bubble foreground all the way up tho.

I intentionally used "foreground" on the more hidden function, and "process" for AccountsDbConfig and the CLI; not sure which way we'll go but I anticipate having to incorporate y'alls feedback and adjusting so I left the mix in

[HaoranYi]

lgtm.

[brooksprumo]

Thanks for this!