(Buglet) job numbers do not get re-used (sometimes)

This is in the category of "not really a bug, but just annoying" and probably also in
"It's been around for so long, that it is probably been elevated into a feature".

The problem is that if you use ^Z a lot (as I do) to create jobs and then put them in the
background, you end up with many jobs as displayed by the "jobs -l" command.  The problem
is that when you then create a new job, it may not get the lowest available job number.

Repro:
$ sleep 1000 &
$ sleep 1000 &
$ sleep 1000 &
$ jobs -l
(displays 3 jobs, numbered 1, 2, 3)
$ kill %1
$ jobs -l
(displays 2 jobs, numbered 2, 3)
$ sleep 1000 &
$ jobs -l
(displays 3 jobs, numbered 2, 3, 4)
$

And, in fact, you can't get a new job to be numbered #1 again unless/until you get rid of
all of them and start over.

I find this annoying because if job #1 exits for some reason (including my accidentally
killing it), I can't get things back to the way they were before (other than by getting
rid of all the jobs and starting over).  In particular, this means I have to re-train
myself to refer to the new job as %4, when I had been used to referring to it as %1.

I would like to see this fixed.

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