CASSANDRA-8032 - User based request scheduler
  - add "user" to RequestSchedulerId enum
  - map it to the current authenticated user in ThriftClientState.getSchedulingValue()
  - in ThriftClientState.getSchedulingValue() throw IllegalStateException when an enum value is not implemented
  - add protected static methods to DatabaseDescriptor to enable testing
  - RequestSchedulerTest checks timeouts, starvation, and throughput with and without weighting