The following cumulative improvements have been made:
- rename `onReduce` and `notifyReduce` to `onFinalReduce` and `notifyFinalReduce`
- add unit test for `SearchShard`
- on* methods in `SearchProgressListener` shouldn't need to be public as they should never be called directly, they only need to be overridden hence they can be made protected. They are actually called directly from a test which required some adapting, like making `AsyncSearchTask.Listener` class package private instead of private
- Instead of overriding `getProgressListener` in `AsyncSearchTask`, as it feels weird to override a getter method, added a specific method that allows to retrieve the Listener directly without needing to cast it. Made the getter and setter for the listener final in the base class.
- rename `SearchProgressListener#searchShards` methods to `buildSearchShards` and make it static given that it accesses no instance members
- make `SearchShard` and `SearchShardTask` classes final

The yaml tests for async search currently sit in its qa folder. There is no reason though for them to live in a separate folder as they don't require particular setup. This commit moves them to the main folder together with the other x-pack yaml tests so that they will be run by the client test runners too.

The following API spec files contain a link to a not-yet-created
async search docs page:

* [async_search.delete.json][0]
* [async_search.get.json][1]
* [async_search.submit.json][2]

The Elaticsearch-js client uses these spec files to create their docs.
This created a broken link in the Elaticsearch-js docs, which has broken
the docs build.

This PR adds a temporary redirect for the docs page. This redirect
should be removed when the actual API docs are added.

[0]: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/async_search.delete.json
[1]: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/async_search.get.json
[2]: https://github.com/elastic/elasticsearch/blob/master/x-pack/plugin/src/test/resources/rest-api-spec/api/async_search.submit.json