Allow extending CompositeType comparator

Quoting Sylvain from CASSANDRA-2474,

if you declare a comparator like CompositeType(IntType, UUIDType, UTF8Type), you can have columns that have only some prefix of the component (typically '42:92d21d0a-d6cb-437c-9d3f-b67aa733a19f' is a valid column name for that comparator), and as a consequence it would be possible to allow 'extend' the preceding comparator to say CompositeType(IntType, UUIDType, UTF8Type, IntType)