Field data diet.

We have lots of unused, or almost unused methods in our field data impls,
especially when dealing with ordinals. Let's nuke them.

Close #5874
Remove abstraction in the percentiles aggregation.

We initially added abstraction in the percentiles aggregation in order to be
able to plug in different percentiles estimators. However, only one of the 3
options that we looked into proved useful and I don't see us adding new
estimators in the future.

Moreover, because of this, we let the parser put unknown parameters into a hash
table in case these parameters would have meaning for a specific percentiles
estimator impl. But this makes parsing error-prone: for example a user reported
that his percentiles aggregation reported extremely high (in the order of
several millions while the maximum field value was `5`), and the reason was that
he had a typo and had written `fields` instead of `field`. As a consequence,
the percentiles aggregation used the parent value source which was a timestamp,
hence the large values. Parsing would now barf in case of an unknown parameter.

Close #5859
Fail a DFS_QUERY_THEN_FETCH request if all shards failed the QUERY phase.

Today, if some shards pass the DFS phase but all of them fail the QUERY phase,
the response will only consist of failed shards. We should throw an exception
instead in order to be consistent with the QUERY_THEN_FETCH type.
Instantiate facets/aggregations during the QUERY phase.

In case of a DFS_QUERY_THEN_FETCH request, facets and aggregations are currently
instantiated during the DFS phase while they only become useful during the QUERY
phase. By instantiating during the QUERY phase instead, we can make better use
of recycling since objects will have a shorter life out of the recyclers.

Close #5821