Implement new Rate Control

Implement new Rate Control

Current implementation of rate control has the potential for bursts and
stalls when iodepth>1 due to feedback delay while IO is pending.  more
description here:
https://docs.google.com/drawings/d/1EG-9eGlNvw-m9m0wMSb_C_lyJcaPlhEFbkHRsVpt4wo/edit?usp=sharing

This commit changes the rate control mechanisms to use feed forward io
issues for rate feedback. Moving the rate control on submissions
instead of completions eliminates feedback delay. More details on the
change here:
https://docs.google.com/drawings/d/1NphdZGjYGuOLWJzvXHv44zuy0nnSunvFrROCVfA67Y8/edit?usp=sharing