Allow gnt-debug submit-job to take multiple args

Currently “gnt-debug submit-job” takes a single argument and has
non-trivial startup-costs; in order to exercise the job system, it is
better to be able to submit multiple jobs with a single invocation of
the script.

This patch extends it to take multiple argument, de-serialize the
opcodes and then submit all of them as fast as possible, in order to
increase pressure on the master daemon.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Alexander Schreiber <als@google.com>