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

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