NOTE: The upgrade is backwards compatible with existing sessions, but
the upgrade of redis-rack v2.1.2 changed Redis keys from
`session:gitlab:<random hex value>` to `session:gitlab:2::<hash of hex
value>`. If a session does not have a key in the new schema, it will be
created transparently. The old session key will eventually be expired
automatically.

To upgrade to rack 2.0.9, we need to do the following:

1. Fix ActiveSession to use new Rack::Session::SessionId
2. Add a monkey patch for ActionController::TestSessionPatch

Controller tests were failing without the changes in
https://github.com/rails/rails/pull/38063, which is available on the
Rails `6-0-stable` branch but not in Rails 6.0.2.2.

3. Remove CGI escaping of ActiveSession keys. This was not needed
because CGI escaping was already being done by Rails.

4. Fix deletion of Rack session keys with ActiveSession

redis-rack v2.1.2 changed the session key from one based on the public
ID to the private ID. We need to adapt ActiveSession to delete both
versions of the key to clear out old data and to make it work with the
redis-rack key name changes.