This commits prepares the database load balancing code for always being
enabled, even when no replicas are configured.

To achieve this, the load balancer now supports the "primary_only"
option. When enabled, the list of replicas is ignored (which is empty
anyway), and a fake replica is used instead. This fake replica simply
reuses the primary connection of the load balancer it belongs to. We
chose this approach as there's simply too much code that assumes at
least one replica is present when load balancing is enabled.

In addition, we make various changes to ConnectionProxy to allow us to
always enable the use of this class. As an example, some tests use code
like this:

    ActiveRecord::Schema.define do
      create_table :foo do
        ...
      end
    end

Internally Rails ends up performing some `respond_to?` checks to expose
certain methods (or not). Without defining a custom
`respond_to_missing?`, these checks return `false` and thus the method
isn't available.

We've also had to change a few more methods in the ConnectionProxy
object to handle forwarding trailing hashes. In the previous setup
certain tests would still trigger deprecation warnings. This can be
avoided by using `...` everywhere, and getting rid of the `sticky:`
keyword argument.

Changelog: changed