Fix gitlab:setup when connections are shared

What does this MR do and why?

This task was originally changed to support multiple databases as part of #352770 (closed)

Fix the gitlab:setup rake task when multiple databases are configured to share the same primary connection. The task currently fails with:

ActiveRecord::StatementInvalid: PG::ConnectionBad: PQconsumeInput() server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.

because it terminates all the connections to main, and then tries to terminate all connections to ci, but the connection to ci has already been terminated in the first step, since ci points at main.

Instead, if ci isn't setup to use database_tasks, we should skip over it.

How to set up and validate locally

Since this task will delete your development db, you can test it against a separate database.

1. Create development and test databases:

   create database foobar_development;
   create database foobar_test;

2. Update your database.yml to point at these (other usual parameters like host, user, password, etc. omitted):

   development:
     main:
       database: foobar_development
     ci:
       database: foobar_development
       database_tasks: false
   
   test:
     main:
       database: foobar_test
     ci:
       database: foobar_test
       database_tasks: false

3. Run the rake task rails gitlab:setup. On master, this will fail with the error:

   ActiveRecord::StatementInvalid: PG::ConnectionBad: PQconsumeInput() server closed the connection unexpectedly
       This probably means the server terminated abnormally
       before or while processing the request.

   But on this branch, it should properly setup the database.

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