Fix InternalSearchHits serialization to be deterministic

The assertion on binary equality for streamable serialization
sometimes fails due to the usage of identify hashmaps inside
the InternalSearchHits serialization. This only happens if
the number of shards the result set is composed of is very high.
This commit makes the serialziation deterministic and removes
the need to serialize the ordinal due to in-order serialization.
Revert "Fix InternalSearchHits serialization to be deterministic"

This reverts commit 8db7b2b20ba332c3b69fb7b686ab6273c5abece0.
[FIX] Indexed scripts should always use script_id.
Update: Detect noop updates sent with doc_as_upsert

This should help prevent spurious updates that just cause extra writing
and cache invalidation for no real reason.

Close #6822
[TEST] Fix indexed script test.
Fielddata: Switch to Lucene DV APIs.

This commits removes BytesValues/LongValues/DoubleValues/... and tries to use
Lucene's APIs such as NumericDocValues or RandomAccessOrds instead whenever
possible.

The next step would be to take advantage of the fact that APIs are the same in
Lucene and Elasticsearch in order to remove our custom comparators and use
Lucene's.

There are a few side-effects to this change:
 - GeoDistanceComparator has been removed, DoubleValuesComparator is used instead
   on top of dynamically computed values (was easier than migrating
   GeoDistanceComparator).
 - SortedNumericDocValues doesn't guarantee uniqueness so long/double terms
   aggregators have been updated to make sure a document cannot fall twice in
   the same bucket.
 - Sorting by maximum value of a field or running a `max` aggregation is
   potentially significantly faster thanks to the random-access API.

Our aggs and p/c aggregations benchmarks don't report differences with this
change on uninverted field data. However the fact that doc values don't need
to be wrapped anymore seems to help a lot. For example
TermsAggregationSearchBenchmark reports ~30% faster terms aggregations on doc
values on string fields with this change, which are now only ~18% slower than
uninverted field data although stored on disk.

Close #6908
[FIX] Fix update parser to accept script_id
[DOCS] Update examples to groovy.
[DOCS] Fix indentation
[DOCS] Fix typo
Fixed typo in AbstractFieldMapper

    similariry -> similarity
Core: allow index.merge.scheduler.max_thread_count to be updated dynamically

Lucene allows the max_thread_count to be updated, but this wasn't
fully exposed in Elasticsearch.

Closes #6925
Test: make sure randomizer doesn't swap in SerialMergeScheduler on us