Priority function specifically for the master.
This doesn't make the super users always priority since that would make everything
to the master into high priority.
Specifically when reporting that a region is in transition master will try and edit the meta
table. That edit will block the thread until successful. However if at the same time meta is
also moving then we need to ensure that the regular region that's moving isn't blocking
processing of the request to online meta. To accomplish this this priority function makes sure
that all requests to transition meta are handled in different threads from other report region
in transition calls.
Returns a 'priority' based on the request type.
Currently the returned priority is used for queue selection.
See the SimpleRpcScheduler as example. It maintains a queue per 'priory type'
HIGH_QOS (meta requests), REPLICATION_QOS (replication requests),
NORMAL_QOS (user requests).