Split handling HTTP requests into separate class

Split handling HTTP requests into separate class

Until now HTTP requests were handled in the same class as incoming
connections (http.server.HttpServer). With this change the request
handling is delegated to a separate class which can be re-used in tests
without creating a socket, etc.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>