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
Aggregations: Remove `ordinals` execution hint.

This was how terms aggregations managed to not be too slow initially by caching
reads into the terms dictionary using ordinals. However, this doesn't behave
nicely on high-cardinality fields since the reads into the terms dict are
random and this execution mode loads all unique terms into memory.

The `global_ordinals` execution mode (default since 1.2) is expected to be
better in all cases.

Close #6499
Fielddata: Don't expose hashes anymore.

Our field data currently exposes hashes of the bytes values. That takes roughly
4 bytes per unique value, which is definitely not negligible on high-cardinality
fields.

These hashes have been used for 3 different purposes:
 - term-based aggregations,
 - parent/child queries,
 - the percolator _id -> Query cache.

Both aggregations and parent/child queries have been moved to ordinals which
provide a greater speedup and lower memory usage. In the case of the percolator
it is used in conjunction with HashedBytesRef to not recompute the hash value
when getting resolving a query given its ID. However, removing this has no
impact on PercolatorStressBenchmark.

Close #6500