Expose task backup stats

[tomwhite]

It would be useful to know how many backup tasks were run (for https://github.com/cubed-dev/cubed-benchmarks/issues/3), probably as a part of TaskEndEvent (number of backups run for the task, and possibly even which was the one that succeeded).

This would require some restructuring since the backup logic is currently separated from the callback logic. For example, in Lithops, backups are managed in map_unordered, but the callbacks are managed in execute_dag, which calls map_unordered.