private static HFileBlock shallowClone(HFileBlock blk, ByteBuff newBuf) {
    return createBuilder(blk, newBuf).build();
  }
Creates a new HFileBlockBuilder from the existing block and a new ByteBuff. The builder will be
loaded with all of the original fields from blk, except now using the newBuff and setting
isSharedMem based on the source of the passed in newBuff. An existing HFileBlock may have been
an {@link ExclusiveMemHFileBlock}, but the new buffer might call for a
{@link SharedMemHFileBlock}. Or vice versa.
@param blk     the block to clone from
@param newBuff the new buffer to use
hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileBlock.java