Add a luxi call for multi-job submit

As a workaround for the job submit timeouts that we have, this patch
adds a new luxi call for multi-job submit; the advantage is that all the
jobs are added in the queue and only after the workers can start
processing them.

This is definitely faster than per-job submit, where the submission of
new jobs competes with the workers processing jobs.

On a pure no-op OpDelay opcode (not on master, not on nodes), we have:
  - 100 jobs:
    - individual: submit time ~21s, processing time ~21s
    - multiple:   submit time 7-9s, processing time ~22s
  - 250 jobs:
    - individual: submit time ~56s, processing time ~57s
                  run 2:      ~54s                  ~55s
    - multiple:   submit time ~20s, processing time ~51s
                  run 2:      ~17s                  ~52s

which shows that we indeed gain on the client side, and maybe even on
the total processing time for a high number of jobs. For just 10 or so I
expect the difference to be just noise.

This will probably require increasing the timeout a little when
submitting too many jobs - 250 jobs at ~20 seconds is close to the
current rw timeout of 60s.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
(cherry picked from commit 2971c913)