check_config should identify loops in proxy mode

[Torxed]

If a vhost is set to proxy mode, and where the proxy target/endpoint is 127.0.0.1:443, slimHTTP will hang indefinitely due to the lack of threads. Since slimHTTP will connect to the proxy (self), and wait for data as it's not part of the main event loop (yet).
Which means that slimHTTP can't process the incoming client request from itself - and thus hang.

the HTTP_PROXY_REQUEST should be part of the main event poller, although I'm not sure how to wrap it in there just yet.
So for now, let check_config find and warn/raise an issue when the proxy config points to a listening address/session of itself.

[Torxed]

The PROXY object should probably behave the same way as STREAM_CHUNKED behaves where the object is yielded and saved in a session, so that data can be fetched/sent in "chunks" if need be.