Deferred Task Mapping

[lightsighter]

Currently invocations of map_task by Legion are required to return a valid mapping. It would be better if there were a way for the mapper to defer the mapping of a task to a future point in time if something is not working out (e.g., memory is not available and needs to be freed up). To this end we could add a new field to MapTaskOutput that is a MapperEvent the mapper can specify to Legion to say when to try the mapping again. The mapper would then be responsible for triggering this event from some other mapper call in order to ensure forward progress. The downside to this is that mappers could create hangs by failing to trigger their deferral events. The benefit though is that mappers wouldn't be stuck waiting for the mapper event directly (which would often be unsafe) and can instead defer the mapping of the task.