Fix CheckpointTool writing only one ssp per task

Currently when using CheckpointTool to write checkpoints, it only writes a checkpoint of a single ssp per task. By debugging the code, looks like the flatMap() on the checkpoint of Optional tuple(taskname -> Map(ssp -> offset)) merges the results by key taskname. So if we have more than one ssps for a task, it will result in only one ssp left after merging. I add a fix by store the results explicitly in a list and then groupBy() on it, which fixed the problem. I still don't understand why scala will merge the key in flatmap.