[BUILD] Add BWC tests to release script

This commit adds the ability to run bwc tests during the release
process to ensure the current release is backwards compatible with
the latest installed previous version. Installed means available
in the configured bwc test path.

Closes #6953
Update scripting.asciidoc

Added an ID for native java scripts
[TEST] Reduce the number of threads in StressUpdateDelete test.
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
Fix connect concurrency, can cause connection nodes to close
Looking at the connect code, if 2 threads at the same time try and connect to a node, and both enter sequentially the connectLock code block, the second one would try and put the connection in the map, and close the replaced channels, which will cause the existing connection to close as well (since it removes the node from the connectedNodes map)
To fix this, simply make sure we properly check the existence of the connection within the connectionLock block, so there won't be concurrent connections going on.
While doing this, also went over all the mutation code that handles disconnections, and made sure they are properly done only within a connection lock.
closes #6964
Unicast discovery: only disconnect from temporary connected nodes
In unicast discovery, we try to reuse existing discovery nodes based on the node address they have. If we find an existing node based on its address, and for some reason its not connected, don't add it to the list of nodes to disconnect from, as that (full) connection is useful down the road
closes #6966
[TEST] Add simple sort assertions for bwc tests

Today we only do count searches to ensure sane results are returned
after upgrading etc. This change adds sorting to the picture asserting
on simple numeric sorting that uses field data etc. after upgrading.

Relates to #6967
PhraseSuggester: Collate option should allow returning phrases with no matching docs

A new option `prune` has been added to allow users to control phrase suggestion pruning when `collate`
is set. If the new option is set, the phrase suggestion option will contain a boolean `collate_match`
indicating whether the respective result had hits in collation.

CLoses #6927