Improve Dolt sysvar support

[timsehn]

There are many MySQL of system variables like @@max_user_connections, @@max_connections, @@port. A full list can be found here:

https://dev.mysql.com/doc/refman/8.0/en/dynindex-sysvar.html

Dolt does not set many of these and for some returns non-sensical values. For instance we always return 3306 for @@port even if you've set a different port (#5898).

We need to run a project tio go through that very long list, set the ones we can, and finally document what we support and do not support under:

https://docs.dolthub.com/sql-reference/sql-support

[timsehn]

Probably first step here is to do read-only support for most of these variables and then do a pass on editing them actually changes the behavior of the running server.

[zachmu]

@@max_connections returns the correct value but changing its value on a running server doesn't change the behavior. You have to set @@persist.max_connections and then restart the server for changes to take effect.