Revert "Get rid of ambiguities about exactly what object is being used to send calls to the scheduler. (See AURORA-42)."

This reverts commit b17bc1f96be9bae0bc5b8d7da8735fcb96772f0d.
Add a test to encourage taking care with thrift changes.

We have more to do w.r.t. automating detection and proper handling of schema
changes (specifically in the scheduler).  However, i see this as low-hanging
fruit to at least serve as a reminder.

Reviewed at https://reviews.apache.org/r/16986/