Disallow converting database to schema for the current database

[vy-ton]

If a user uses ALTER DATABASE... CONVERT TO SCHEMA WITH PARENT... for the database they are currently connected to, CockroachDB allows it which causes a bad UX after the database is deleted

root@127.0.0.1:63916/rafi> ALTER DATABASE rafi CONVERT TO SCHEMA WITH PARENT my_app;
TODO (rohany): Implement

Time: 124.378ms

warning: error retrieving the database name: pq: database "rafi" does not exist

root@127.0.0.1:63916/?> show databases;
  database_name
-----------------
  bill
  defaultdb
  movr
  my_app
  postgres
  system
  vy
(7 rows)

Time: 2.71ms

warning: error retrieving the database name: pq: database "adam" does not exist
root@127.0.0.1:63916/?> show search_path;
ERROR: database "adam" does not exist
SQLSTATE: 3D000
warning: error retrieving the database name: pq: database "adam" does not exist

Desired behavior
Instead, we should disallow this command for the current database and error out

[blathers-crl]

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