Implemented array subscript lookup in JSON::Object::find.

Review: https://reviews.apache.org/r/26634
Fixes necessary due to JSON::Array::values change to std::vector.

Review: https://reviews.apache.org/r/26635
Mark running tasks killed during framework shutdown.

When a framework is shut down e.g. by calling driver.stop() from the
scheduler, running tasks are marked KILLED before migrating them to
completed.

Review: https://reviews.apache.org/r/25250