This patch changes the generic "multiple job executor" to use the many
jobs submit model, which automatically makes all its users use the new
model.

This makes, for example, startup/shutdown of a full cluster much more
logical (all the submitted job IDs are visible fast, and then waiting
for them proceeds normally).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>