private ByteBuff getCachedHeader(final long offset) {
      PrefetchedHeader ph = this.prefetchedHeader.get();
      return ph != null && ph.offset == offset ? ph.buf : null;
    }
Check atomic reference cache for this block's header. Cache only good if next read coming
through is next in sequence in the block. We read next block's header on the tail of reading
the previous block to save a seek. Otherwise, we have to do a seek to read the header before
we can pull in the block OR we have to backup the stream because we over-read (the next
block's header).
@see PrefetchedHeader
@return The cached block header or null if not found.
@see #cacheNextBlockHeader(long, ByteBuff, int, int)
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java