Replace use of Priority in the Scheduling infrastructure with an opaque ShedulerRequestKey

Based on discussions in YARN-4888, this jira proposes to replace the use of Priority in the Scheduler infrastructure (Scheduler, Queues, SchedulerApp / Node etc.) with a more opaque and extensible SchedulerKey.

Note: Even though SchedulerKey will be used by the internal scheduling infrastructure, It will not be exposed to the Client or the AM. The SchdulerKey is meant to be an internal construct that is derived from attributes of the ResourceRequest / ApplicationSubmissionContext / Scheduler Configuration etc.