Database::Connection#exists? would call `connection` to determine if
the database exists. Without load balancing enabled, this would call
`ActiveRecord::Base.connection`, which in turn calls
`ActiveRecord::Base.retrieve_connection`. If no connection could be
established or the database doesn't exist, this will raise an error.

When using the load balancer, the `Connection#connection` method instead
just returns a `ConnectionProxy`, without running any database queries.
The result is that `exists?` would return `true` even if the database
didn't exist.

To work around this, `Connection#exists?` obtains the database version
and caches this using Rails' schema cache. This ensures we run an actual
database query the first time, without running a query _every_ time.
Using the schema cache means we don't need to implement our own caching
logic.

Changelog: fixed