Update the ThreadPoolTaskRunner to use thread priorities

 * 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
there shouldn't be a need for JsonIgnore, Druid's Jackson defaults to requiring explicit annotations for all fields