fix(celery): Fix dropped transactions under Celery 4.2+

[untitaker]

• Work around Custom headers not propagated as documented ? celery/celery#4875 which causes us to lose transaction events. Fix Trace propagation is broken under Celery 4.4 #824
• Rewrite celery testsuite to use redis backend and test transactions too. This is better because it works on more celery versions (memory backend is often broken). However, this still does not trigger the bug, so I guess for this to be properly tested we'd need to install rabbitmq into CI? No thanks