[WAL] Merged region should get its WAL according to WALProvider.

  HRegion createMergedRegionFromMerges(final HRegionInfo mergedRegionInfo,
      final HRegion region_b) throws IOException {
    HRegion r = HRegion.newHRegion(this.fs.getTableDir(),
        this.getWAL(), // Here should use rsService.getWAL(mergedRegionInfo), then wal provider and strategy if there is will take effect.
        fs.getFileSystem(), this.getBaseConf(), mergedRegionInfo,
        this.getTableDesc(), this.rsServices);
    ...

    return r;
  }