Apply minimum cache TTL of 5 seconds, and remove publish intent auto-cleanup.

This avoids hammering origin around the time of publication.

Applying this minimum cache TTL means that there's no
pressing need to clean these up immediately.

This behaviour previously meant that it was important that any "coming
soon" content item was published before the intent.  Removing this
behaviour simplifies this use case, and also simplifies the code.

Intents will now only be cleaned up by the nightly cronjob.
Fair points.  Updated and rebased.