Disallow_DML from readwritesplit or create a new router type for readonly

Customers often confuse READ_ONLY on the database with what maxscale can actually do. They think that Maxscale can be used to create a read_only listener/service combination.

It would not be difficult to add this feature. Readwritesplit router will block DML when it sees that all servers are currently with global READ_ONLY=ON. The warning on Maxscale is:

2024-03-11 12:32:11   warning: (11131) [readwritesplit] Write query received from edward@192.168.8.211. 'delayed_retry_timeout' exceeded before a primary could be found. Closing client connection.

This goal could be achieved by adding a variable in readwritesplit (such as disallow_dml=true), or by adding a whole new router. The expected behavior would be to send a warning or error to the client when a DML command is sent, but not disconnect the session.

An added benefit would be to prevent DML under any circumstances, even for accounts with SUPER privilege.