Safe activities are:
- those which are not visible outside current transaction (IOW, queued
  in activity buffer)
- those which are already marked as failed, as no activity node will try
  to process these
Other activities may already be running.
Note: deleting even failed activities may violate activity dependencies
(other activities may be waiting on the ones being deleted, even those
not spawned yet), but if this is an issue then some other code should
take care of these as well.
Also, rework a CMFActivity test checking that activties already-spawned on
a later-deleted context get discarded instead of being executed: previous
implementation was working around the activity deletion removed in this commit.
Also, test indexation/unindexation parallelism.