Application restart introduced.

When processing a restart request, the router sends a QUIT message to all
existing processes of the application.  Then, a new shared application port is
created to ensure that new requests won't be handled by the old processes of
the application.

src/nxt_router.h

@@ -125,6 +125,8 @@ struct nxt_app_s {
     uint32_t               max_pending_processes;
     uint32_t               max_requests;
 
+    uint32_t               generation;
+
     nxt_msec_t             timeout;
     nxt_msec_t             idle_timeout;