Term Vectors: support for distributed frequencies

Adds distributed frequencies support for the Term Vectors API. A new parameter
called `dfs` is introduced which defaults to `false`.
I think the whole client isn't necessary, you can just do this instead:
```java
this.searchRequest = new SearchRequest(indices).types(types).source(new SearchSourceBuilder().query(boolBuilder));
```

Then you can also remove the client field in ShardTermVectorService.java.