Improve and simplify late data logic.
Fix persistence bug.
Merge pull request #871 from carueda/platform_agent_R2C4

integration of rollup status handling in the platform agent.
Bump pyon
instead of a single event subscriber for all children (for a given event type),
create them per child to explicitly indicate the origin and avoid need for
event  filtering at reception.
call .stop instead of .deactivate for termination of event subscribers