Docs: Update execution hint docs for Significant terms agg

copied over the relevant pieces from the terms agg

Closes #8532
Core: Added query/filter wrapper that builds the actual query to be executed on the last possible moment to aid with index aliases and percolator queries using `now` date expression.

Percolator queries and index alias filters are parsed once and reused as long as they exist on a node. If they contain time based range filters with a `now` expression then the alias filters and percolator queries are going to be incorrect from the moment these are constructed (depending on the date rounding).

 If a range filter or range query is constructed as part of adding a percolator query or a index alias filter then these get wrapped in special query or filter wrappers that defer the resolution of now at last possible moment as apposed during parse time. In the case of the range filter a special Resolvable Filter makes sure that `now` is resolved when the DocIdSet is pulled and in the case of the range query `now` is resolved at query rewrite time. Both occur at the time the range filter or query is used as apposed when the query or filter is constructed during parse time.

Closes #8474
Closes #8534
[BLOOM] Fix Bloom filter ram usage calculation

BloomFilter actually returned the size of the bitset as the
size in bytes so off by factor 8 plus a constant :)

Closes #8564
Update DiskThresholdDecider javadoc

Since v1.3.0, and issue #6201, the default values in code and documentation have been updated to 85% and 90% for low and high watermarks. However, the related javadoc still contains the initial values : this commit fix this.
Aggregations: Fix geohash grid doc counts computation on multi-valued fields.

Close #8512
Configuration: Tab characters in YAML should throw an exception.

Throw an exception if there is a 'tab' character in the elasticsearch.yml file

Close #8259
fix of Bulk update child doc, NPE error message when parent is not specified #8365
  - Throw an RoutingMissingException instead of NPE
Bulk indexing issue - missing parent routing causes NullPointerException.
Now each error is reported in bulk response rather than causing entire bulk to fail.
Added a Junit test but the use of TransportClient means the error is manifested differently to a REST based request - instead of a NullPointer the whole of the bulk request failed with a RoutingMissingException. Changed TransportBulkAction to catch this exception and treat it the same as the existing logic for a ElasticsearchParseException - the individual bulk request items are flagged and reported individually rather than failing the whole bulk request.

Closes #8365
Parser throws NullPointerException when Filter aggregation clause is empty.
Added Junit test that recreates the error and fixed FilterParser to default to using a MatchAllDocsFilter if the requested filter clause is left empty.
Also added fix and test for the Filters (with an "s") aggregation.

Closes #8438
Fix wrong error messages in MultiMatchQueryParser.

Also fix a typo in the comment

Close #8597
Fix a typo in a javadoc comment in MapperService.

Close #8575
Docs: Added new elasticsearch-river-kafka plugin to the documentation (uses latest version of Kafka, EL Bulk API, and supports concurrent requests)

Closes #8518
Docs: Remove reference to imaginary "no_docs_query"

No reference to it in the source code except this file.

Closes #8566
Docs: Update update-settings.asciidoc

Inconsistent indentation

Closes #8525
Fix: If dangling_timeout was set to 0 and auto_import_dangled
was set to yes, dangling indices were deleted by mistake,
because a RemoveDanglingIndices runnable was added
to every dangling indices, without considering the auto_import_dangled
setting.
fixed NPE in LocalGatewayMetaState
Docs: Update plugins.asciidoc on river plugins section

Adding links to Amazon S3 and Google Drive river plugins

Closes #8544
Docs: Reword note regarding _source for accuracy

Previously it suggested _source was always present, when that is not the case.

Closes #8491
[GEO] Fix for ArithmeticException[/ by zero] when parsing a "polygon" with one pair of coordinates

While this commit is primariy a fix for issue/8433 it adds more rigor to ShapeBuilder for parsing against the GeoJSON specification. Specifically, this adds LinearRing and LineString validity checks as defined in http://geojson.org/geojson-spec.html to ensure valid polygons are specified. The benefit of this fix is to provide a gate check at parse time to avoid any further processing if an invalid GeoJSON is provided.  More parse checks like these will be necessary going forward to ensure full compliance with the GeoJSON specification.

Closes #8433
Correcting coordinate checks on LinearRing and LineString, updating test