MEDIUM: session: implement the "use-server" directive

Sometimes it is desirable to forward a particular request to a specific
server without having to declare a dedicated backend for this server. This
can be achieved using the "use-server" rules. These rules are evaluated after
the "redirect" rules and before evaluating cookies, and they have precedence
on them. There may be as many "use-server" rules as desired. All of these
rules are evaluated in their declaration order, and the first one which
matches will assign the server.
diff --git a/src/proto_http.c b/src/proto_http.c
index 3f6ec04..9af3f69 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -6221,7 +6221,7 @@
 				 * empty cookies and mark them as invalid.
 				 * The same behaviour is applied when persistence must be ignored.
 				 */
-				if ((delim == val_beg) || (t->flags & SN_IGNORE_PRST))
+				if ((delim == val_beg) || (t->flags & (SN_IGNORE_PRST | SN_ASSIGNED)))
 					srv = NULL;
 
 				while (srv) {