Currently we don't fetch a new batch of oplog entries until we have finished applying the previous batch. Instead, we should fetch oplog entries as fast as we can get them from our sync source and write them down, and in parallel we should be applying oplog entries as fast as we can. This would be especially useful in conjunction with SERVER-32513, so that when coming out of initial sync you never stop syncing from your sync source, which would greatly limit the degree to which initial sync was dependent on the size of its sync source's oplog.