[capi] get rid of legacy request body API (will be replaced later) and make capi compilation cmake-optional
[capi] compile capi examples only when configured so (disable body demo for now)
[base] CompositeSource: code readability improvements
[base] BufferRefSource: API cleanup
[http] HttpStatus: introduce Hangup (499), the nginx-invented response status code to indicate early client connection hangups
TODO file update
ChangeLog file updates
tests: app_cluster.db updates
[http] major rework/simplification on how HTTP request bodies are processed.

This should also be how other major HTTP (proxy) servers handle HTTP
request bodies and should help me getting rid of complexity
to further improve stability.