Add a helper function to the RAPI client

This adds a new method WaitForJobCompletion that can be used for
client who are not interested in the entire job log, just in its
completion status.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>