maxCatchUps

The partitions the node reads behind their head at once, all its subscriptions together; 0 for no bound. A partition whose subscriptions start behind the head (a history migrate-data imported, a long stop) waits for one, and the others stand at their checkpoints, so the node keeps its store for its commands while it catches up. More do not catch up sooner on one store, and raise the latency of the commands meanwhile.