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_application.h

@@ -60,8 +60,6 @@ struct nxt_common_app_conf_s {
 
     char       *working_directory;
 
-    uint32_t   workers;
-
     union {
         nxt_python_app_conf_t  python;
         nxt_php_app_conf_t     php;