void readKey(ByteBuffer in, int keyLength, int valueLength) {
    readKey(in, keyLength, valueLength, 0, null);
  }
Analyze the key and fill the state. Uses mark() and reset() in ByteBuffer.
@param in          Buffer at the position where key starts
@param keyLength   Length of key in bytes
@param valueLength Length of values in bytes
hbase-common/src/main/java/org/apache/hadoop/hbase/io/encoding/CompressionState.java