(bug 27965) Paging in list=categorymembers was completely broken. It was paging by...

(bug 27965) Paging in list=categorymembers was completely broken. It was paging by cl_from alone, while the index is on (cl_to, cl_type, cl_sortkey, cl_from) and only cl_to is constant. Fixed by paging on (type, sortkey, from), but using type|from|sortkey for clcontinue so any pipe characters in the sortkey are easier to handle. This needs the schema change in r83812 to work correctly, otherwise rows with cl_type=file will be skipped in certain cases.