[DOCS] document simple_query_string's `lenient` option
function_score: remove explanation of query score from functions

The score is explained already, it should not be again explained per function.

Also, remove explanation from parameter list of ScoreFunction#explainScore()
and leave only the score.

This also removes ExplainableSearchScript which is not used anywhere and
was the only reason to have the Explanation in the parameter anyway.

closes #7245
Docs: Document distance type and sort mode for many to many geo_points

closes #7280
Tests: tone down random compression tests

Closes #7314
[TEST] Don't invoke RoutingNodes.assertShardStats() via asserts many times in tests, just once on each reroute call.
Core: switch to fixed thread pool by default for management threads

Switch management threads to a fixed thread pool with up to 5 threads, and queue size of 100 by default, after which excess incoming requests are rejected.

Closes #7318

Closes #7320
Cli: parsing multiple times throws `AlreadySelectedException`

This issue has been fixed in commons-cli:1.3 project which sadly has not been released yet.
See https://issues.apache.org/jira/browse/CLI-183

This patch builds another list of options with no selected groups by default.

When commons-cli:1.3 will be released, we need to remove this patch.

Closes #7282.
Aggregations: Fixes pre and post offset serialisation for histogram aggs

Changes the serialisation of pre and post offset to use Long instead of VLong so that negative values are supported.  This actually only showed up in the case where minDocCount=0 as the rounding is only serialised in this case.

Closes #7312