Prohibit non admin writes to system spaces

Summary

Prohibit DML queries into system space for all users; For user=ADMIN_ID we prohibit only a particular tables including:

    const PROHIBITED_SPACES: &[ClusterwideTable] = &[
        ClusterwideTable::Table,
        ClusterwideTable::Index,
        ClusterwideTable::User,
        ClusterwideTable::Privilege,
        ClusterwideTable::Routine,
    ];

Refactor dml_prohibited check in cas.rc

Adjust tests according to new behaviour, for regular users on write into system space we get error AccessDenied

• Close #402 (closed)
• Cherry-pick to: none
• Docs follow-up: not necessary