Add more stringent DDL and DML permission checks to the system table.

[buzzers]

Feature Request

Is your feature request related to a problem? Please describe:
Executing a DDL statement in the system database causes the DDL execution queue to hang, and all subsequent DDL statements are permanently waiting.

Describe the feature you'd like:
When executing DDL and DML statements in the system database, you should check permissions and reject any "write" statements, such as CREATE TABLE.

Teachability, Documentation, Adoption, Migration Strategy:
Some permission checks have been added to DDL and DML statements in #15095, but CREATE and DROP have been ignored. According to the comments, it is for compatibility considerations.
In practice, however, executing CREATE and DROP statements in the system database results in the DDL queue locking, so there is no need to check for compatibility.