Replace JoinInfo namedtuple with Join class

[Anssi Kääriäinen]

The rationale for this change is that by using a class we can implement all kinds of different joins (LATER JOINS over subquery) if needed for special cases. In addition, this way feels cleaner to me as the join generation logic now happens in a dedicated class instead of "hard-coding" it to SQLCompiler.

Implementation in ​https://github.com/django/django/pull/3561

[Anssi Kääriäinen]

This should be ready for commit, see PR for review details.

[Tim Graham <timograham@…>]

In ab89414f40db1598364a7fe4cfac1766cacd2668:

Fixed #23853 -- Added Join class to replace JoinInfo

Also removed Query.join_map. This structure was used to speed up join
reuse calculation. Initial benchmarking shows that this isn't actually
needed. If there are use cases where the removal has real-world
performance implications, it should be relatively straightforward to
reintroduce it as map {alias: [Join-like objects]}.

[GitHub <noreply@…>]

In 7acbe89:

Refs #23853 -- Updated sql.query.Query.join() docstring.

Follow up to ab89414f40db1598364a7fe4cfac1766cacd2668.

[Tim Graham <timograham@…>]

In 31d7fc85:

[1.11.x] Refs #23853 -- Updated sql.query.Query.join() docstring.

Follow up to ab89414f40db1598364a7fe4cfac1766cacd2668.

Backport of 7acbe89cc2a72f1b85d415c1cdb622c0dbbbe37c from master