CDC and $paxos tables are managed internally by Scylla. Users are
already prohibited from running ALTER and DROP commands on CDC tables.
In this commit, we extend the same restrictions to $paxos tables to
prevent users from shooting themselves in the foot.
Other commands are generally allowed for CDC and $paxos tables. An
important distinction is that CDC tables are meant to be accessed
directly by users, so appropriate permissions must be set for
non-superusers. In contrast, $paxos tables are not intended for direct
access by users. Therefore, this commit explicitly disallows
non-superusers from accessing them. Superusers are still allowed
access for debugging and troubleshooting purposes.
Note that these restrictions apply even if explicit permissions have
been granted. For example, a non-superuser may be granted SELECT
permissions on a $paxos table, but the restriction above will
still take precedence. We don't try to restrict users
from giving permissions to $paxos tables for simplicity.
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