More Like This Query: Switch to using the multi-termvectors API

The term vector API can now generate term vectors on the fly, if the terms are
not already stored in the index. This commit exploits this new functionality
for the MLT query. Now the terms are directly retrieved using multi-
termvectors API, instead of retrieving the texts using the multi-get API. The
terms are then directly passed to Lucene MLT.
sorry for the misleading comment, here is where we lose headers and request context, the items don't need them, only the main request. This one should be created passing in the original request, but that would be `MoreLikeThisRequest` not multi_get. Actually I'm confused on why we still have multi_get items but we don't use multi_get internally anymore