Refining Association Requests

[groue]

This pull request fixes a known issue with associations.

It is now possible to include or join an association several times in the definition of a single request:

// 1. Start from all books with their author...
var request = Book.including(required: Book.author)

// 2. Then filter on Spanish authors...
request = request.joining(required: Book.author.filter(Column("countryCode") == "ES"))

// 3. Then sort by author name and then book title.
let authorAlias = TableAlias()
request = request
    .joining(optional: Book.author.aliased(authorAlias))
    .order(authorAlias[Column("name")], Column("title"))

See the new "Refining Association Requests" chapter in AssociationBasics.md for more information and intended use cases.