Fix OperationContext lifetime management in ReshardingOplogApplier and use batchSize > 1

It is possible for the future returned by ReshardingDonorOplogIterator::getNext() to only become ready after opCtx would have been destructed. This won't happen while ReshardingDonorOplogIterator::_waitForNewOplog() always returns an immediately ready future, but would become an issue as soon as the returned future wasn't immediately ready. It isn't practical for ReshardingDonorOplogIterator::_waitForNewOplog() to always return an immediately ready future because the donor iterator may reached the end of the oplog buffer and need to wait for an insert notification from the ReshardingOplogFetcher.