reset-to-waiting when prerequisite tasks have been removed

[hjoliver]

I want to reset the prerequisites of a whole sub-graph of tasks so that they all run again in the right order (as opposed to triggering all the tasks immediately). [Note I can't explicitly refer to a sub-graph yet - see #1314 - but I can achieve the same thing by resetting an appropriately defined family]. However, if I do this I might find that everything stalls because the prerequisites of the task(s) at the top of the sub-graph(s) have already been removed from the suite.

This may be confusing to users: the (removed) tasks did in fact already run, so why are the associated prerequisites not satisfied? To understand this you have to understand the life cycle of task proxies, but as per #2143 users should not need to know that.

How to do what the user wants:

If a group of one or more tasks gets "reset to waiting", analyse the prerequisites of each one. If a prerequisite is a member of the group, reset it to not-satisfied. If it is not a member of the group and the associated task proxy has been removed from the suite, interrogate the database to see if it already completed (and set the prerequisite satisfied if it did complete).

[hjoliver]

Closed by spawn-on-demand #3515