It looks like `date_nanos` fields weren't likely to work properly in
composite aggs because composites iterate field values using points and
we weren't converting the points into milliseconds. Because the doc
values were coming back in milliseconds we ended up geting very confused
and just never collecting sub-aggregations.

This fixes that by adding a method to `DateFieldMapper.Resolution` to
`parsePointAsMillis` which is similarly in name and function to
`NumberFieldMapper.NumberType`'s `parsePoint` except that it normalizes
to milliseconds which is what aggs need at the moment.

Closes elastic#53168