Added put_length() to get_length() and unpack_key() to pack_key().
Keys were packed with the minimum size of the length field for the key part and 
unpacked with length size of the base column. 
For the purpose of optimal key packing we have the method pack_key(), while rows are 
packed with pack(). Now keys are unpacked with unpack_key() and no longer with 
unpack() which is used for rows.