Switch job IDs to numeric

This has been a long-standing cleanup item, which we've always
refrained from doing due to the high estimated effort needed.

In reality, it turned out that after some infrastructure improvements
(the previous patches), the actual job queue-related changes are quite
small.

We will need to update the NEWS file later, but so far the RAPI
documentation doesn't mention that the job ID is a string (it only
says it is "a number"), so it doesn't look like it needs update.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>