Refactor JoinDependency and friends so that a JoinAssociation can produce an arbitrary number of joins, which will be needed in order to support nested through associations.

performance improvement based on discussion at http://github.com/rails/rails/commit/fbd1d306b95cc2efb6422e12d26d5818a3a42343

Credit goes to all the participants in the discussion

* for simple cases like User.last and User.order('name desc').last no need to perform Array#join operation.

* Instead of performing String#blank? do Array#empty?

* no need to create variable relation