Extend gnt-debug with more debugging options

This patch extends gnt-debug to be able to submit multiple copies of the
input jobs and job contents, in order to simplify testing. It also adds
a timing mode, and splits the execution into separate submit and
execution stages (for timing purposes).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>