@Override
  public String toString() {
    return FastDiffDeltaEncoder.class.getSimpleName();
  }
Copies the first key/value from the given stream, and initializes decompression state based
on it. Assumes that we have already read key and value lengths. Does not set
{@link #qualifierLength} (not used by decompression) or {@link #prevOffset} (set by the calle
afterwards).
hbase-common/src/main/java/org/apache/hadoop/hbase/io/encoding/FastDiffDeltaEncoder.java