Fix bug on unordered keys in prefetch
Fix bug on unordered keys in prefetch

The cql query created had a wrong format for the partition keys.
Occurred when the keys in the classfield were unordered
compared to the CQL create statement, for instance
by first setting a clustering key before the partition key.

bsc-dd/hecuba,bsc-dd/hecuba,bsc-dd/hecuba,bsc-dd/hecuba