Use thread priorities. (aka set `nice` values for background-like tasks)

There are some items which seem to cause resource contesting when they are running. This is an attempt to allow the OS / JVM to help figure out which tasks should be run realtime, and which can be interrupted.

 * 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 Task.Priority enum
   * Can be either NORMAL or LOW
 * 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
same here, let's define this as a default constant