Rename PlacementSet and SchedulingPlacementSet

Currently, the SchedulingPlacementSet is very confusing. Here're its responsibilities:

1) Store ResourceRequests. (Or SchedulingRequest after YARN-6592).
2) Decide order of nodes to allocate when there're multiple node candidates.
3) Decide if we should reject node for given requests.
4) Store any states/cache can help make decision for #2/#3