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.
Maybe use `Lucene.VERSION` instead of hardcoding 4.9?