Execution of execution sets is now fully functional.

However, the server is currently not notified after every program is
successfully executed; it only gets an execution set's results (status
and files) after the whole execution set is executed.

This means two things:
1. the client does not show execution progress whithin an execution set
   anymore,
2. the exec. loop cannot execute an execution set S2 depending on some
   results in an other one (S1) before the latter (S1) is finished,
   even if the results the former set (S2) is depending on are produced
   in early steps during the execution of S1.

Point 1. will be fixed soon.

Point 2. can happen due to the way the SimpleWorkflowPlanner builds
execution sets.  Solving it means that the platform must be able to
send partial results to the server.