Implement background batch processing for sitemap updates instead of locking

[Piedone]

Is your feature request related to a problem? Please describe.

Sitemaps are stored in a single document, thus concurrent updates to it (what happens if you click Publish on two content items at the same time) will cause an exception, see #15743. To overcome this, #15777 introduced locking. While this fixes the immediate issue, for sites where content items are published rapidly, this may result in degraded performance when saving content items.

Describe the solution you'd like

Implement a task-based background processing like what we have for indexing. However, this needs to be checked, because while this can reduce the latency of content publishing, it may increase overall load on the DB and thus reduce throughput.

Describe alternatives you've considered

Locking and background batch processing are the only two solutions I can think of.