Allowing cross db transactions and joins between clusterwide and main

What does this MR do and why?

With the switch to the cells 1.0 architecture and further, we don't need to prevent the cross joins and cross database transactions between gitlab_main_clusterwide and gitlab_main_cell anymore. Which makes it easy for developers to write efficient queries.

Allowing cross db transactions and joins between clusterwide and main

Changelog: other

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Write a test that joins between users and namespaces. Something like this

User.joins("namespaces on users.id = namespaces.id").all # A random query that joins two tables from different schemas.

it should not fail anymore