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.