Minor Description
While instrumenting the INPUT_SUPERSTEP and watching various runs, I see the input split list generated every time a worker calls reserveInputSplit is, for all intents and purposes, immutable per job. Therefore, we can save a fair amount of memory by not re-creating the list and re-querying ZooKeeper on each pass to claim another split. Only the reserved and finished children lists are ever mutated during the input phase of the job.