Write straight into the store directory when Splitting and Merging

As discussed with zhangduo on HBASE-26079, we think there's no actual need for Splits/Merges to make use of temp directories, because the split/merge region files would not be loaded until the new regions are ready to open, meaning the split/merge operation has completed. This would then avoid the need for a FS rename at the end of split/merge operation.