Make read_only a session level variable

Being able to force the session into read-only mode would make it easier to be certain that no operation will write to the database. With stored procedures, it is often very difficult to know whether it's safe to execute on a replica or not.

By making read_only a session level variable, it can be used with SET STATEMENT which would make it possible to call stored procedures with a high level of certainty that it won't cause a write to be made.

Currently the tx_read_only variable can be used to emulate this to some extent but it still won't prevent a write transaction from being done.