[Test] ElasticsearchIntegrationTest.clearDisruptionScheme should test if the current cluster is internal

When running on a non-internal cluster the function is a noop.
[Test] change the default port base for ClusterDiscoveryConfiguration.UnicastZen to 30000

The previous value of 10000 collided with the standard test cluster ports when 6 or more JVMs are used.
fix typo
Internal: Removing unused methods/parameters.

Close #7474
Update reverse-nested-aggregation.asciidoc

Fixed reverse nested example

Closes #7463
[Test] RecoveryBackwardsCompatibilityTests.testReusePeerRecovery used `gateway.recover_after_nodes:3` but may start only a 2 node cluster
[Tests] add proper error message when BWC test fail due to node.local=true  system property

System properties are typically set via the command line and therefore override the node settings. If one has `node.local=true` or `node.mode=local` it can result in cryptic error messages during the test run.
Use Flake IDs instead of random UUIDs when auto-generating id field

Flake IDs give better lookup performance in Lucene since they share
predictable prefixes (timestamp).

Closes #7531

Closes #6004

Closes #5941
[Tests] add proper error message when BWC client creation fail due to node.local=true  system property

System properties are typically set via the command line and therefore override the node settings. If one has `node.local=true` or `node.mode=local` it can result in cryptic error messages during the test run.
[Tests] ExternalCluster change error message when use local network mode due to wrong system properties
[TEST] Add simple test to test RT Lucene IW settings
[TEST] Use a large threshold to prevent relocations in RecoveryBackwardsCompatibilityTests
[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