MDEV-9247: Make default_master_connection settable globally

[abhishek593]

Change default_master_connection scope from SESSION_ONLY to SESSION_VAR. This allows the variable to be set globally, so that new connections automatically inherit the default master connection name.

Changes:

• Updated scope macro in sql_vars.cc.
• Added a new test in default_master_connection_basic.test and updated results of existing tests.

[ParadoxV5]

I’d self-assign, but… global @@default_master_connection might have implications, so I’ll leave it to the JIRA-assigned senior.

By the way, I recall it’s a known design (limitation?) that changes to global variables are not reflected on session variables.

[abhishek593]

Yes, that is the standard behaviour for SESSION_VAR. I used that to allow for a global default for new connections, while also having session isolation.