public long allocate() {
      assert freeCount > 0; // Else should not have been called
      assert sizeIndex != -1;
      ++usedCount;
      long offset = baseOffset + (freeList[--freeCount] * itemAllocationSize);
      assert offset >= 0;
      return offset;
    }
Allocate a block in this bucket, return the offset representing the position in physical
space
@return the offset in the IOEngine
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketAllocator.java