private boolean freeListContains(int blockNo) {
      for (int i = 0; i < freeCount; ++i) {
        if (freeList[i] == blockNo) return true;
      }
      return false;
    }
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