Make parsing of uri and body parameters consistent with single term vector api

uri parameters were not all parsed for the multi term vector request. This commit
makes sure that all parameters are parsed and used when creating the requests for the
multi term vector request.

In order to simplify both code and json request, the request structure now allows
two ways to use multi term vectors:

1. Give all parameters for each document requested in the docs array like this:

```
{
   "docs": [
      {
         "_index": "testidx",
         "_type": "test",
         "_id": "2",
         "terms": [
            "fox"
         ],
         "term_statistics": true
      },
      {
         "_index": "testidx",
         "_type": "test",
         "_id": "1",
         "terms": [
            "quick",
            "brown"
         ],
         "term_statistics": false
      }
   ]
}
```

2. Define a list of ids and give parameters in a separate parameters object like this:

```
{
   "ids": [
      "1",
      "2"
   ],
   "parameters": {
      "_index": "testidx",
      "_type": "test",
      "terms": [
         "brown"
      ]
   }
}
```

uri parameters are global parameters that are set for both cases. They are overwritten
by parameter definitions in the body.

Also, this commit adds the missing setParent(..) and setPreference(..) to TermVectorRequestBuilder.
[DOC] add doc for multi term vector api

closes #3998