Bring the many 'copied' task trigger mechanisms into the task API

Loosely related to MB-26831

A number of tasks exist which have similar trigger mechanisms,

• BgFetcher::run(GlobalTask *task)
• ItemPager::run
• ActiveStreamCheckpointProcessorTask::run

The atomic 'notified' logic should be moved up to a base-class (maybe GlobalTask) along with the wake mechanism, then the task's run function can be 'wrapped' in GlobalTask run who performs the common steps which exist to prevent lost wakeups.

bool BaseTask::run() {

  snooze(INT_MAX);

  notified = false;

  bool rv = derived::run();

  if (derived::moreWork()) {  // note derived::moreWork would check the queue in ActiveStreamCheckpointProcessorTask

     wakeUp();

  }

  return rv;

}

We can also consider a way to clearly implement/separate triggered tasks from periodic tasks with some restructuring of GlobalTask->PeriodicTask->*Task