Don't auto-create connection if nil on methods

Unless told otherwhise, when this proxy class intercepts methods, it will create
a connection if one does not already exist.

https://github.com/zendesk/active_record_host_pool/blob/v0.11.0/lib/active_record_host_pool/pool_proxy.rb#L107-L108

In Rails 5.2.0, they added the following functionality which caused issues with
code mentioned above
https://github.com/rails/rails/blob/v5.2.3/activerecord/lib/active_record/railtie.rb#L180-L195

Here's the associated PRs
rails/rails#28057
rails/rails#31221

This patch overrides the methods, retrieves the connection if it's active and
calls super without creating a new one.

This resolves an issue for applications upgrading to Rails 5.2.3 whereby
connections would be created and thrown away during Rails.application.initialize!

lib/active_record_host_pool/pool_proxy.rb

@@ -84,6 +84,28 @@ def clear_reloadable_connections!
       _clear_connection_proxy_cache
     end
 
+    def release_connection(owner_thread = Thread.current)
+      p = _connection_pool(false)
+      return unless p
+
+      p.release_connection(owner_thread)
+    end
+
+    def flush!
+      p = _connection_pool(false)
+      return unless p
+
+      p.reap
+      p.flush(-1)
+    end
+
+    def discard!
+      p = _connection_pool(false)
+      return unless p
+
+      p.discard!
+    end
+
     private
 
     def rescuable_errors