Introducing extended app process management.

- Pre-fork 'processes.spare' application processes;
- fork more processes to keep 'processes.spare' idle processes;
- fork on-demand up to 'processes.max' count;
- scale down idle application processes above 'processes.spare' after
  'processes.idle_timeout';
- number of concurrently started application processes also limited by
  'processes.spare' (or 1, if spare is 0).

src/nxt_port.h

@@ -158,6 +158,9 @@ struct nxt_port_s {
     nxt_queue_link_t    app_link;   /* for nxt_app_t.ports */
     nxt_app_t           *app;
 
+    nxt_queue_link_t    idle_link;  /* for nxt_app_t.idle_ports */
+    nxt_msec_t          idle_start;
+
     nxt_queue_t         messages;   /* of nxt_port_send_msg_t */
     nxt_thread_mutex_t  write_mutex;