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
@fjy it seems about half the tasks need to be min priority and Normal. in AbstractTask it has the default as normal.

The reason is that the MIN priority on linux systems actually sets the nice value, whereas NORMAL leaves it untouched. If MIN is the default, it might cause future tasks to behave unexpectedly if someone is unaware this is occurring (wondering why their thread keeps getting bumped out compared to other non-executor-managed threads).