cli.JobExecutor: Use counter for indexing jobs

If “SubmitPending” were mixed with calls to “QueueJob”, jobs in the
internal structures will get duplicate indices. With this change each
queued job is assigned a unique index, which will be used for sorting
the results.

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