Refactor term vector api

This is necessary to allow adding a mult term vector request
Multi term vector request
--------------------------

This feature allows to retrieve [term vectors](https://github.com/elasticsearch/elasticsearch/issues/3114) for a list of documents. The json request has exactly the same [format](https://github.com/elasticsearch/elasticsearch/issues/3484) as the ```_termvectors``` endpoint

It use it, call

```
curl -XGET 'http://localhost:9200/index/type/_mtermvectors' -d '{
    "fields": [
        "field1",
        "field2",
        ...
    ],
    "ids": [
        "docId1",
        "docId2",
        ...
    ],
    "offsets": false|true,
    "payloads": false|true,
    "positions": false|true,
    "term_statistics": false|true,
    "field_statistics": false|true
}'

```

The return format is an array, each entry of which conatins the term vector response for one document:

```
{
   "docs": [
      {
         "_index": "index",
         "_type": "type",
         "_id": "docId1",
         "_version": 1,
         "exists": true,
         "term_vectors": {
         	...
         }
      },
      {
         "_index": "index",
         "_type": "type",
         "_id": "docId2",
         "_version": 1,
         "exists": true,
         "term_vectors": {
         ...
         }
      }
   ]
}
```

Note that, like term vectors, the mult term vectors request will silenty skip over documents that have no term vectors stored in the index and will simply return an empty response in this case.

Closes #3536
Added extra logging
improve search while create test
- improve the test to be more re-creatable
- have tests for various number of replica counts, to check if failures are caused by searching on replicas that might not have been refreshed yet
- improve test to test explicit index creation, and index creation caused by index operation
- have an initial search go to _primary, to check if failure fails when searching on replica because it missed a refresh