Add ability to do a CB update at a specified time

With the current configuration backend model using config-fetch-wait-time, it is difficult if not impossible to synchronize changes across multiple servers, or to do "timed" deployments of changes.

I recommend the implementation of time-based config updates:

{
  "command": "remote-server-update-at",
  "arguments": {
    "server-tags": [ "headquarters" ]
    "reconfig-at": [
      { "time": "<timespec>",
        "date": "<datespec>"
      }
    ],

  "service": [ "dhcp4" ]
}

The provided JSON is probably illegal/syntactically incorrect. It is provided to give an idea of what's rattling around in my brain.

Third (and last!) issue created from #846 (closed)