@Override
    public String toString() {
      return "offset=" + this.offset + ", header=" + Bytes.toStringBinary(header);
    }
Data-structure to use caching the header of the NEXT block. Only works if next read that comes
in here is next in sequence in this block. When we read, we read current block and the next
blocks' header. We do this so we have the length of the next block to read if the hfile index
is not available (rare, at hfile open only).
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java