[DOCS] Javadoc: delete index request does not support empty String

In 0.90.x I was able to delete all my indices from the java api by calling

    client.admin().indices().prepareDelete(new String[] {}).execute().actionGet();

However this fails in 1.0.0 with

org.elasticsearch.action.ActionRequestValidationException: Validation Failed: 1: index / indices is missing;
	at org.elasticsearch.action.ValidateActions.addValidationError(ValidateActions.java:29)
	at org.elasticsearch.action.admin.indices.delete.DeleteIndexRequest.validate(DeleteIndexRequest.java:72)
	*snip long stacktrace*

which points me to

    public ActionRequestValidationException validate() {
        ActionRequestValidationException validationException = null;
        if (indices == null || indices.length == 0) {
            validationException = addValidationError("index / indices is missing", validationException);
        }
        return validationException;
    }

So that's what now throws the error, however the documentation still says:

    /**
     * Deletes an index based on the index name.
     *
     * @param indices The indices to delete. Empty array to delete all indices.
     */
    DeleteIndexRequestBuilder prepareDelete(String... indices);

Closes #5164.
Closes #5167.
Closes #5168.
Fix SearchContext from being closed prematurely

Fixes SearchContext from being closed during initialization or immediately
after processing is started

Closes #5165
Add test and randomization for #5165
Alias code cleanup

- Renamed IndexMetaData#removerAlias to removeAlias
- Removed IndexTemplateMetaData#fromXContentStandalone unused method (relates to #4511)
- MetaDataIndexAliasesService fix typo in comment
- Alias removed unused constructor that accepts both alias name and filter
[TEST] Added tests for alias filter that is valid json but not a valid filter
Added script to extract release notes

Just call ./es_release_notes.pl <issue-tag> to get all release notes.

By default html output is returned, but you can switch to markdown by calling
./es_release_notes.pl <issue-tag> markdown
[DOCS] Fixed typo in create index api
[TEST] Initialization errors are checked before returning "no tests to run" error

If there initialization errors and no tests to execute at the same time, better to return the initialization errors, whose check should be first then, as it might be that the "no tests to execute" is caused by the initialization errors.
Added tests for empty token chars on ngram tokenizer

This relates to #5120
Recovery API

Adds a new API endpoint at /_recovery as well as to the Java API. The
recovery API allows one to see the recovery status of all shards in the
cluster. It will report on percent complete, recovery type, and which
files are copied.

Closes #4637
Merge branch 'issue-4637.1' of github.com:aleph-zero/elasticsearch into issue-4637.1