Multiple Gateways - optimize reloading

[sjberman]

As a followup to the multiple gateway support story, we need to optimize how many config reloads and messages we are sending to agent.

In the initial implementation, any change results in NGF updating and sending every config (even if it's irrelevant). We should only send configuration to agent if it's changed.

Acceptance

• NGF only sends configuration to an agent if the corresponding NGINX instance needs to be changed.
• NGF only calls the NGINX Plus API if upstreams have changed

Dev Notes

• Check the updateNginxConf workflow section in the internal design doc for details on how we can do this
• Process() no longer needs to return the ChangeType variable.