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
Tests: Add more assertions to ExistsMissingTests.
Internal: Don't let `took` be negative.

`took` is computed based on the system clock and can be negative if the clock
time was updated during the execution of the search request. This commit
protects against these cases by replacing `took` with 1 if the elapsed time is
negative.

Close #7968