This move the `murmur3` field to the `mapper-murmur3` plugin and fixes its
defaults so that values will not be indexed by default, as the only purpose
of this field is to speed up `cardinality` aggregations on high-cardinality
string fields, which only requires doc values.
I also removed the `rehash` option from the `cardinality` aggregation as it
doesn't bring much value (rehashing is cheap) and allowed to remove the
coupling between the `cardinality` aggregation and the `murmur3` field.
Close#12874
David Pilato
Adrien Grand