Changelog note for #228

Changelog note for #228
- Merge PR #228 : infra-keep-probing option to probe hosts that are
  down.  Add infra-keep-probing: yes option. Hosts that are down are
  probed more frequently.
  With the option turned on, it probes about every 120 seconds,
  eventually after exponential backoff, and that keeps that way. If
  traffic keeps up for the domain. It probes with one at a time, eg.
  one query is allowed to probe, other queries within that 120 second
  interval are turned away.