* Defaults the thread priority to java.util.Thread.NORM_PRIORITY in io.druid.indexing.common.task.AbstractTask
* Each exec service has its own Task Factory which is assigned a priority for spawned task. Therefore each priority class has a unique exec service
* Added priority to tasks as taskPriority in the task context. <0 means low, 0 means take default, >0 means high. It is up to any particular implementation to determine how to handle these numbers
* Add options to ForkingTaskRunner
* Add "-XX:+UseThreadPriorities" default option
* Add "-XX:ThreadPriorityPolicy=42" default option
* AbstractTask - Removed unneded @JsonIgnore on priority
* Added priority to RealtimePlumber executors. All sub-executors (non query runners) get Thread.MIN_PRIORITY
* Add persistThreadPriority and mergeThreadPriority to realtime tuning config
