Core: Improve XContentBuilder API.
Internal: refactored TransportSingleCustomOperationAction, subclasses and requests

TransportSingleCustomOperationAction is subclassed by two similar, yet different transport action: TransportAnalyzeAction and TransportGetFieldMappingsAction. Made their difference and similarities more explicit by sharing common code and moving specific code to subclasses:
- moved index field to the parent SingleCustomOperationAction class
- moved the common check blocks code to the parent transport action class
- moved the main transport handler to the TransportAnalyzeAction subclass as it is only used to receive external requests through clients. In the case of the TransportGetFieldMappingsIndexAction instead, the action is internal and executed only locally as part of the user facing TransportGetFieldMappingsAction. The corresponding request gets sent over the transport though as part of the related shard request
- removed the get field mappings index action from the action names mapping as it is not a transport handler anymore. It was before although never used.

Closes #7214
[DOCS] Added section describing how to return only agg results

Closes #5875
[TEST] Fix SimpleQueryTests.testRangeQuery assumptions.

This test assumed that the `num` field was mapped as an integer on all shards
and thus that all of them should fail when providing a timezone. However, since
it used dynamic mappings, some shards might have this field not mapped, as a
consequence they didn't fail.
Mapping: fixes dynamic mapping of geo_point fields

If a dynamic mapping for a geo_point field is defined and the first document specifies the value of the field as a geo_point array, the dynamic mapping throws an error as the array is broken into individual number before consulting the dynamic mapping configuration.  This change adds a check of the dynamic mapping before the array is split into individual numbers.

Closes #6939
[Build] @Nullable annotation is not applied to primitive types.
[Build] Make Eclipse aware of Elasticsearch's Nullable annotation for null pointer dereference analysis.
Internal: Fix a very rare case of corruption in compression used for
internal cluster communication.

See CorruptedCompressorTests for details on how this bug can be hit.
This change also removes the ability to use the unsafe variant of
ChunkedEncoder, removing support for the compress.lzf.decoder setting.
Core: Pass down the types from the delete mapping request to the delete by query request.

The `.percolator` type is a hidden type and therefor the types from the delete mapping request should passed down to the delete by query request, otherwise the percolator type gets ignored and the percolator queries don't get deleted from disk (only unregistered).

Closes #7087
Internal: adjusted visibility of GetRequest members (from protected to private) and resolved warning
[DOCS] fixed DeleteIndexedScriptResponse javadocs
Core: Avoid null references that may be returned due to concurrent changes or inconsistent cluster state

Closes #7181
Mappings: Make sure that multi fields are serialized in alphabetic order to ensure that the source is always the same.

Closes #7215
Test: don't need to wait for concrete mappings since we map both fields when we create the index
Internal: adjusted TermVectorRequest serialization to not serialize and de-serialize the index twice

Closes #7221
Geo: fixes geo_shapes which intersect dateline

If a geo_shape had edges which either ran vertically along the dateline or touched the date line but did not cross it they would fail to parse.  This is because the code which splits a polygon along the dateline did not take into account the case where the polygon touched but did not cross the dateline.  This PR fixes those issues and provides tests for them.

Close #7016
Geo: Better error for invalid multipolygon

Closes #7126
Fix for failing BasePolygonBuilder
Internal: changed every single index operation to not replace the index within the original request

An anti-pattern that we have in our code, noticeable for java API users, is that we modify incoming requests by replacing the index or alias with the concrete index. This way not only the request has changed, but all following communications that use that request will lose the information on whether the original request was performed against an alias or an index.

Refactored the following base classes: `TransportShardReplicationOperationAction`, `TransportShardSingleOperationAction`, `TransportSingleCustomOperationAction`, `TransportInstanceSingleOperationAction` and all subclasses by introduced an InternalRequest object that contains the original request plus additional info (e.g. the concrete index). This internal request doesn't get sent over the transport but rebuilt on each node on demand (not different to what currently happens anyway, as concrete index gets set on each node). When the request becomes a shard level request, instead of using the only int shardId we serialize the ShardId that contains both concrete index name (which might then differ ffrom the original one within the request) and shard id.

Using this pattern we can move get, multi_get, explain, analyze, term_vector, multi_term_vector, index, delete, update, bulk to not replace the index name with the concrete one within the request. The index name within the original request will stay the same.

Made it also clearer within the different transport actions when the index needs to be resolved and when that's not needed (e.g. shard level request), by exposing `resolveIndex` method. Moved check block methods to parent classes as their content was always the same on every subclass.

Improved existing tests by randomly introducing the use of an alias, and verifying that the responses always contain the concrete index name and not the original one, as that's the expected behaviour.

Added backwards compatibility tests to make sure that the change is applied in a backwards compatible manner.

Closes #7223
[TEST] don't use multiple names (e.g. aliases) pointing to the same concrete index when using indexRandom

indexRandom will try to delete bogus documents multiple times since they get tracked by indexOrAlias/id, and after the actual deletion any other attempt throws error and fails the test