kapacitor has a new topic store format and command

**ETA:** Next kapacitor release, ~15 February_
**PR:** https://github.com/influxdata/kapacitor/pull/2742
**Issue:** https://github.com/influxdata/kapacitor/issues/2723

The topic store format in kapacitor has been changed to allow incremental updates. This should significantly speed
up saving topic state, allowing higher performance under high alert loads.

When a version of kapacitor with this new format is started, it will automatically upgrade the original topic store format
to the new version.

There is a command `kapacitor downgrade` which will restore the topic store format to the original version and then exit
(because the new kapacitor cannot run with the original topic store format).

In addition, this new format permits alert states to be deleted when an alert resets/recovers. When an alert changes from
any other status to OK, the alert state will be deleted from the topic store. This will only be visible when kapacitor restarts,
because the in-memory history of alerts is not modified, only the on-disk history read at startup.

kapacitor 1.7.0 (?) not sure what we will number it. The next release.

Start by reviewing the linked pull request and the related Kapacitor issue to confirm the released behavior and terminology. Identify the appropriate Kapacitor documentation entry point, then document the new topic store format, automatic upgrade, kapacitor downgrade command, and alert-state deletion behavior. Done means the documented behavior matches the released implementation.

Written by the indexing model from the issue text.