Change producer_run() to run a specific producer in more cases

Found while tracking down TS-3105. It seemed like sometimes producers were having producer_run called on them multiple times. producer_run can be run on a particular producers or on all producers added at that point in time.

There are a couple places where multiple producers are set up and so producer_run(NULL) makes sense. But in many cases a producer is carefully added and then producer_run(NULL) is called on all producers that are around that that point in time.

It seems safer to call producer_run() on the newly added producer in those cases.