Map<Long, Long> getSwapSegmentBySizeMap() {
    return swapSegmentNumBySize;
  }
@return return the map kept in this DiskSpaceAllocator to record total number of swap segments by size. Here, total
number of swap segments include both segments in use and segments available in reserve pool.
ambry-store/src/main/java/com/github/ambry/store/DiskSpaceAllocator.java