Term Vectors: support for realtime

By default term vectors are now realtime, as opposed to previously near
realtime. If they are not found in the index, they will be generated on the
fly. The document is fetched from the transaction log and treated as an
artificial document. One can set `realtime` parameter to `false` in order to
disable this functionality. This consequently makes the MLT query realtime in
fetching documents, as it previsouly used to be before switching from using
the multi get API to the mtv API.

Closes #7846