[TEST] remove global scope mention from ElasticsearchIntegrationTest#buildTestCluster

The global cluster gets created from a static block and shared through all tests in the same jvm. The `buildTestCluster` method can't get called passing in `Scope.GLOBAL`, hence removed its mention from it as it might be misleading. The only two scopes supported within the `buildTestCluster` method are `SUITE` and `TEST`.
[Test] NoMasterNodeTests.testNoMasterActions mistakenly assuming index will be created on bulk

This test indexes into an into existing index after loosing master. Before this meant an attempt to auto create this index. With the new light rejoin we do not clean the cluster meta data upon loss of master. This means the test doesn't try to auto create it but fails on a no master block
[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.
[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
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
[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
Improve node type documentation

When discussing "smart load balancers", we are calling them non-data nodes and only specifying node.data: false as the setting.  Would be better to call them client nodes and provide the common configuration of not just setting node.data: false, but node.data:false + node.master:false.  Also including information on dedicated master nodes here for completeness.
[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.