While merging relations preserve context for joins

Fixes #3002. Also see #5494.

```ruby
class Comment < ActiveRecord::Base
  belongs_to :post
end

class Author < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :author
  has_many :comments
end
```

`Comment.joins(:post).merge(Post.joins(:author).merge(Author.where(:name => "Joe Blogs"))).all` would
fail with `ActiveRecord::ConfigurationError: Association named 'author' was not found on Comment`.

It is failing because `all` is being called on relation which looks like this after all the merging:
`{:joins=>[:post, :author], :where=>[#<Arel::Nodes::Equality: ....}`. In this relation all the context that
`Post` was joined with `Author` is lost and hence the error that `author` was not found on `Comment`.

Ths solution is to build JoinAssociation when two relations with join information are being merged. And later
while building the arel use the  previously built `JoinAssociation` record in `JoinDependency#graft` to
build the right from clause.

Thanks to Jared Armstrong (https://github.com/armstrjare) for most of the work. I ported it to make it
compatible with new code base.

/cc @jonleighton