A participant that stores the workitem in the same storage used by the
engine and the worker(s).

              part = engine.register_participant 'alfred', Ruote::StorageParticipant
              # ... a bit later
              puts "workitems still open : "
              part.each do |workitem|
                puts "#{workitem.fei.wfid} - #{workitem.fields['params']['task']}"
              # ... when done with a workitem
                # this will remove the workitem from the storage and hand it back
                # to the engine

Does not thread by default (the engine will not spawn a dedicated thread
to handle the delivery to this participant, the workitem will get stored
via the main engine thread and basta).