Aggregations: Fix reducing of range aggregations.

Under some rare circumstances:
 - local transport,
 - the range aggregation has both a parent and a child aggregation,
 - the range aggregation got no documents on one shard or more and several
   documents on one shard or more.
the range aggregation could return incorrect counts and sub aggregations.

The root cause is that since the reduce happens in-place and since the range
aggregation uses the same instance for all sub-aggregation in case of an
empty bucket, sometimes non-empty buckets would have been reduced into this
shared instance.

In order to avoid similar bugs in the future, aggregations have been updated
to return a new instance when reducing instead of doing it in-place.

Close #6435