Ability to query CB for changes that will be made on next update

A feature request to create a way to query CB for the changes that are to be applied on the next update...

This request (called "remote-server-update-changes" here, but feel free to change it) called with a particular server-tag would return all changes that are pending to be applied to the given server.

Changes being made to server-tag "all" would be included in the response.

{
  "command": "remote-server-update-changes",
  "arguments": {
    "server-tags": [ "headquarters" ]
  },
  "service": [ "dhcp4" ]
}

This is one of three issues created from #846 (closed)