Cache dateViewFields()

Follow-up to #2604516: Improve the row render function

Problem/Motivation

Follow-up from #2600416: Fix date_views_fields() call in row plugin preRender().

We added a function dateViewFields() in CalendarHelper, which figures out what the view fields are that can contain datetime information. It's a quick port from the date_view_fields() function that was available in the date_view module.

The current implementation is painfully slow. We should add cache tags.

Proposed resolution

Add the cache tags.

Remaining tasks

Do it.