Support publishing to a load balancer

Steps to reproduce

1. Configure load balancer with
   • two DX core public instances with JCR cluster
   • sticky sessions (or make sure multiple request of one publication operation are redirected to the same public instance)
2. Point the only receiver to the load balancer
   • https://nightly.magnolia-cms.com/.magnolia/admincentral#app:configuration:browser;/modules/publishing-core/config/receivers/magnoliaPublic8080@url:treeview: (this is Magnolia 6.2 settings in JCR, or the microprofile config in Magnolia 6.3)
   • so the two receivers are hidden behind the load balancer
3. Publish a content from author instance.

Expected results

The content is successfully published / can be retrieved from both public instances

Actual results

The COMMIT operation might end up on different public instance than the publication operation so the publication is not completed.

Workaround

Set the receiver URL directly to the public instance URL without the load balancer.

Development notes

The transactional publishing runs two operations

• publication
• commit

with the same headers except for one header:

mgnlexchangeaction: publish

mgnlexchangeaction: commit