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`.
Since you only gather the dfs it make more sense to me to free the search context after getting the dfs in the first phase. Now because the free search context is cleared as part of the reduce in moveToSecondPhase(), clearing the search context will introduce additional remote calls to the nodes where the dfs is fetched from.