[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
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
get field mapping: return _boost and _analyzer by their default names also

_boost and _analyzer mapping can now be retrieved by their default name
or by given name ("path" for _analyzer, "name" for _boost)
Both still appear with their default name when just retrieving the mapping.
(see SimpleGetFieldMappingTests#testGet_boostAnd_analyzer and
     SimpleGetMappingTests#testGet_boostAnd_analyzer)

The index_name handling is removed from _boost. This never worked anyway,
see this test: https://github.com/brwe/elasticsearch/commit/36450043640f49f959d953dfd546f33606cb953a

Change in behavior:

_analyzer was never a field mapper. When defining an analyzer
in a document, the field (_analyzer or custom name) was indexed
with default string field properties. These could be overwritten by
defining an explicit mapping for this field, for example:

```
PUT testidx/doc/_mapping
{
  "_analyzer": {
    "path": "custom_analyzer"
  },
  "properties": {
    "custom_analyzer": {
      "type": "string",
      "store": true
    }
  }
}
```
Now, this explicit mapping will be ignored completely, instead
one can only set the "index" option in the definition of _analyzer
Every other option will be ignored.

Reason for this change:
The documentation says

"By default, the _analyzer field is indexed, it can be disabled by settings index to no in the mapping."

This was not true - the setting was ignored. There was a test
for the explicit definition of the mapping (AnalyzerMapperTests#testAnalyzerMappingExplicit)
but this functionallity was never documented so I assume it is not in use.