Store the load balancer proxy in ActiveRecord

What does this MR do?

Related to !64321 (merged) / #211875 (closed)

Currently we are storing this in @proxy in Gitlab::Database::LoadBalancing. This class is auto-loaded which means it is also unloaded when a Ruby change is detected. Not sure why this wasn't unloaded with the classic autoloader.

This moves this to a class instance variable in ActiveRecord::Base

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Security

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