Scheduler: throttle thread queueing by CUs

[apfitzge]

Problem

• Scheduler queues up more work than necessary which can lead to more work being queued due to account conflicts

Summary of Changes

• CU limits are now more reasonable, with much lower estimated/actual ratios than the previous scheduler
• Can now use CUs to throttle the scheduled work queues so we don't queue up too much work

Fixes #

[tao-stones]

Looks good idea to limit number of TXs to be scheduled at once. Just few small questions.

[tao-stones]

lgtm - thanks for clarifying the intent

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.