Add possibility to choose primary source for schemarouter

When a Schemarouter query the database , epecially information_schema,
it is not determined, which server will be queried.

According the the shown tables and databases,
where the source is information_schema.schemata and show databases,
the output is different , depending which server will be queried.

select @@hostname, @@server_id, t.* from information_schema.schemata t; 

In addition Maxscale choose different server in Maxscals 2.4 and 2.5/6
in the automatic selection process.

Adding a simple command should give the user more control, which server should be used.

A workaround is using hintfilter

select @@hostname, @@server_id, t.* from information_schema.schemata t; – maxscale route to server 2

https://mariadb.com/kb/en/mariadb-maxscale-6-hintfilter/#hintfilter

But this not works for all usecases.

Adding a new command to choose the primary server for schemarouter override the automatic selection.

alter service schemarouter primary_server server2

This gives more controil over the schemarouter behaviour.