Add ability to dynamically adjust workload as it's running

I've been in several situations now where I've wanted to change the rate limit of a running job without stopping it entirely. Sometimes it would be nice to just reduce the throughput by 10% or change the read rate.

When starting a job, add an option to open up a control port. My current thinking is to use gRPC to listen on that control port, and add a `client` mode to easy-cass-stress that would connect to that control port and accept commands to adjust the workload.

Start by reading the easy-cass-stress entry point and how a job is started. Investigate the proposed gRPC control port and client mode, including commands for changing throughput or read rate while the job runs. Done means a running job can accept those adjustments without stopping.

Written by the indexing model from the issue text.