The oplog applier batcher batches according to certain rules and passes these batches to the POA applier. However, the applier doesn't necessarily honor batch boundaries. In some cases, it does, but this is by peering into the batch and checking if it's there's an oplog entry type that requires the drain+inline path. That basically resembles the original batching logic, and so maybe we don't need an OplogApplierBatcher at all - the POA applier can just be fed a stream of oplog entries instead and can dispatch them.