MXS-1725 Use session_set_response() in Cache

- session_set_response() made const correct
- set_response() function added to mxs::FilterSession; calls
  session_set_response().
- Cache uses set_response() for delivering the cache result
  to the client.

server/core/session.cc

@@ -1229,7 +1229,7 @@ bool session_remove_variable(MXS_SESSION* session,
     return removed;
 }
 
-void session_set_response(MXS_SESSION *session, MXS_UPSTREAM *up, GWBUF *buffer)
+void session_set_response(MXS_SESSION *session, const MXS_UPSTREAM *up, GWBUF *buffer)
 {
     // Valid arguments.
     ss_dassert(session && up && buffer);