Fixes #795 - Inconsistent behaviour when using ORDER BY and LIMIT while updating the graph

* Changes priority of ORDER BY...LIMIT (PlanBuilder.TopX) so it is executed
  before updating the graph
* Changes so TopPipeBuilder waits for aggregation, but not for return
  expressions execution
* UpdateActionBuilder waits on slice to be done before accepting work
* SliceBuilder does not wait for updates to be done