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.c

@@ -29,6 +29,7 @@ nxt_port_mp_cleanup(nxt_task_t *task, void *obj, void *data)
 
     nxt_assert(port->use_count == 0);
     nxt_assert(port->app_link.next == NULL);
+    nxt_assert(port->idle_link.next == NULL);
 
     nxt_assert(nxt_queue_is_empty(&port->messages));
     nxt_assert(nxt_lvlhsh_is_empty(&port->rpc_streams));