Allow enabling rate controller from settings, update docs

[jwhitlock]

The rate controller is working well in production with these settings:

• rate_controller_target = 1,500,000
• rate_controller_enabled = 1
• rate_controller_kp = 0.4
• rate_controller_ki = 0.002
• rate_controller_kd = 0.0

It should be possible for settings, pulled from environment variables, to set the defaults, rather than manually set them in Redis.

When this is done, it would make sense to remove the "beta" label from the rate controller, and update the tuning recommendations.