Schedule @aBlock to run async in a coordinated
     fashion with other scheduled tasks.

     We want to give the current process a chance
     to finish setting up handlers, and other work, that
     the current process is performing.
     Therefore, the submission block is submitted at
     a priority 1 less than the current process.  Once the block
     is activated, the process priority is changed to the requested
     @taskPriority and rescheduled to run @aBlock

     We also want to gaurantee that the submissions of tasks
     are executed in the order in which they were submitted.
     For this, we use the ProcessScheduler queue to help maintain
     that order.  Rather than a normal 'fork' of the submission block,
     which places it at the front of the queue...we instead call #addToReadyLast:
     to ensure the submissions line up in a queue like fashion.

     @Note: To maintain the properties listed above, primarily where task
     submission should be at a lower priority level than the current caller process,
     an exception is thrown if the caller is running in a systemBackgroundPriority (1)
     process.  To work around this issue, fork off a new process of at least
     userBackgroundPriority (2) to schedule the task.

     Arguments:
        aBlock - <Block>
     Raises:
        <EsAsyncScheduleTaskError>