Recover and restarts should cascade through prerequisites.

[natacha-beck]

In a big graph of tasks, triggering a recover or restart
on a task should reset the task status downstream in
a cascading fashion, based on the prerequisites.

Unfortunately, a task does not normally record what its downstream
tasks are, in the general CBRAIN framework (PsomSubtasks do,
but that's because I coded them to store teh dowstream
dependencies in their params).

There's an added complication when tasks are actually managed
by higher-level meta-tasks, such as Parallelizers and
Serializer: these should be discovered and reset too.