GET requests should go to primary shard

GET requests should be routed to the primary shard via the `preference` parameter, to prevent consistency issues.  By default, Elasticsearch will round-robin GET requests between all available shards (primaries and replicas).  Updates to a document are only consistent from the perspective of the thread updating the document, to all other threads it is possible to see a stale copy while the update is "in-flight" to one of the replicas.

By setting `preference=_primary`, the GET request is forced to route to the primary shard.  Since the primary is the source of truth, it is guaranteed to always be consistent.  You can [read more about preference here](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-preference.html).

Just as a note, when you add/remove aliases in a single operation, it is an atomic operation.  E.g. if you do this:

```
POST /_aliases
{
    "actions": [
        { "remove": { "index": "my_index_v1", "alias": "my_index" }},
        { "add":    { "index": "my_index_v2", "alias": "my_index" }}
    ]
}
```

The alias swaps from `my_index_v1` to `my_index_v2` atomically.  You can read more about swapping aliases [here](https://www.elastic.co/guide/en/elasticsearch/guide/current/index-aliases.html) and [here](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-aliases.html).  If you were using this method and still seeing consistency issues, that's a bug...it'd be great if you can open a ticket! :)

Author: polyfractal

src/IndexRotator.php

@@ -86,7 +86,8 @@ public function getPrimaryIndex()
 		$primaryPayload = [
 			'index' => $this->configurationIndexName,
 			'type' => static::TYPE_CONFIGURATION,
-			'id' => static::PRIMARY_ID
+			'id' => static::PRIMARY_ID,
+			'preference' => '_primary'
 		];
 		try {
 			$primary = $this->engine->get($primaryPayload);