Remove all of the hacks and support infrastructure around Rack sessions. Rails provides this by default so we don't need it for 90% of users. The other 10% should know and provide a Rack session.

This is a big change and has the potential to break many installs. It will be part of the 7.0 major version bump and require a lengthy beta period to ensure we document as many edge cases and solutions as possible.

See also #4671, #4728 and many others.

* Remove HEAD aliases which don't work
* Add simplest possible heartbeat HEAD which returns 200 or 500.