Architecture variant: multiple backends

[rfc2822]

Architecture variant where we have more backends on the WebDAV server side

• more complicated to implement and set up on WebDAV server side
• reduced redundancy (by topic support)
• Redirector doesn't have complex responsibility
• higher overall complexity, making it less probable that everything will be implemented and working
• in this variant we also have an SSE backend for clients that don't support another push method (because they for instance can't afford to provide a push service or expect that one is available – and such a push server would be required for Web Push or UnifiedPush)

[rfc2822]

Closing because this is not our current favorite architecture. Please see #21 for our current favorite (and reasoning) and comment there if you have feedback, which is very welcome.