YARN-1051 introduced a ReservationSytem that enables the YARN RM to handle time explicitly, i.e. users can now "reserve" capacity ahead of time which is predictably allocated to them. Currently, the ReservationSystem currently has an implicit FIFO priority. This JIRA tracks effort to generalize this to arbitrary priority. This is non-trivial as the greedy nature of our ReservationAgents might need to be revisited if not enough space if found for late-arriving but higher priority reservations.