Allow HttpBackends to make use of interal HttpServer threads.

HttpBackends which can do multi-threading to increase performance
can now be notified about the HttpServer server-threads to install
their own thread-local handlers.
HttpClient, allow for manual initialisation of the internal state

Needed for HttpBackends/Transports which receive the HTTP header in
a parsed fashion incompatible with HTTP itself.
Implemented a FastCGI HttpBackend for writing FCGI applications

Tested with Lighttpd and nginx