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.