Remove depreacted finders

They were deprecated in 4.0, planned to remove in 4.1
Merge pull request #11153 from strzalek/remove-ar-deprecated-finders

Remove depreacted finders
Fix rake test_sqlite3_mem

Clearly nobody uses this except me. It's fast people!
Fix test

Oops. We need to estalish/remove the connection in the setup/teardown,
else it messes with the fixtures.
Apply default scope when joining associations.

For example:

    class Post < ActiveRecord::Base
      default_scope -> { where published: true }
    end

    class Comment
      belongs_to :post
    end

When calling `Comment.join(:post)`, we expect to receive only
comments on published posts, since that is the default scope for
posts.

Before this change, the default scope from `Post` was not applied,
so we'd get comments on unpublished posts.