[Engine] Verify checksums on merge

Enable lucene verification of checksums on segments before merging them.
This prevents corruption from existing segments from silently slipping into
newer merged segments.

Closes #7360
typo s/removeDistruptionSchemeFromNode/removeDisruptionSchemeFromNode
[Analysis] Add missing docs for latvian analysis
[Internal] Do not use a background thread to disconnect node which are remove from the ClusterState

After a node fails to respond to a ping correctly (master or node fault detection), they are removed from the cluster state through an UpdateTask. When a node is removed, a background task is scheduled using the generic threadpool to actually disconnect the node. However, in the case of temporary node failures (for example) it may be that the node was re-added by the time the task get executed, causing an untimely disconnect call. Disconnect is cheep and should be done during the UpdateTask.

Closes #7543
Aggregations: Only return aggregations on the first page when scrolling.

Aggregations are collection-wide statistics so they would always be the same.
In order to save CPU/bandwidth, we can just return them on the first page.

Same as #1642 but for aggregations.
Aggregations: Forbid usage of aggregations in conjunction with search_type=SCAN.

Aggregations are collection-wide statistics, which is incompatible with the
collection mode of search_type=SCAN since it doesn't collect all matches on
calls to the search API.

Close #7429
Packaging: Add default oracle jdk 7 (x64) path in debian init script

On Debian amd64, oracle jdk .deb packages made using make-jpkg (from
java-package) default to /usr/lib/jvm/jdk-7-oracle-x64.

Closes #7312
Internal: remove unused constructor and adjust methods visibility in DelegatingActionListener
Java api: remove needless copy constructor from DeleteIndexedScriptRequest
Internal: fix members visibility, remove unused constant and needless try catch in indexed scripts transport actions
Java api: change base class for GetIndexedScriptRequest and improve its javadocs

`GetIndexedScriptRequest` now extends `ActionRequest` instead of `SingleShardOperationRequest`, as the index field that was provided with the previous base class is not needed (hardcoded).

Closes #7553
[TEST] resolved warning in IndexedScriptTests
_boost: Fix "index" setting

Serialization if "index" setting for boost did not work since
the serialization was just true/false instead of valid options
"no"/"not_analyzed"/"analyzed".

closes #7557
[TEST] replaced assert with actual assertions in TemplateQueryTest
[TEST] added test to verify GetIndexedScriptRequest serialization after recent changes
Aggregations: Fixes resize bug in Geo bounds Aggregator

Closes #7556
[Docs] Fix incorrect eclipse m2e markdown syntax

Closes #7546.
[docs] fix typo in language analyzer docs
Core: Remove DocSetCache.

This class was unused.

Close #7582
Internal: deduplicate useful headers that get copied from REST to transport requests

The useful headers are now stored into a `Set` instead of an array so we can easily deduplicate them. A set is also returned instead of an array by the `usefulHeaders` static getter.

Relates to #6513

Closes #7590