Rearrange db_clause defines and add operator short versions.
Check for DB_CLAUSE_UNKNOWN in db_clause_set_type(), return error if someone tries to set it to that.
Generate clause functions for all object fields and add a get by clauses.
This can be used to fetch custom lists of objects.
Add tests for clause functions and get by clauses.
Change the ds gone command function change_keys_retracted_to_unsubmitted() to use the new clause/get_by_clauses functions.