Rename client -> worker in rate controllers

[nklincoln]

There is an ongoing rename of clients to workers within Caliper.

There is currently lingering references to client within the rate controllers; this should be modified for:

• variables
• properties passed to the controller
• docs for the rate controllers

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[aklenik]

I guess this will be resolved by #943 soon