Task thread priority

 * 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

Supposedly: on Oracle JVM this is only activated when setting `-XX:+UseThreadPriorities` and running the JVM as root, or setting `-XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=42` (where 42 is any number neither 0 nor 1). See [this thread](http://www.akshaal.info/2008/04/javas-thread-priorities-in-linux.html)