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_main_process.c

@@ -114,12 +114,6 @@ static nxt_conf_map_t  nxt_common_app_conf[] = {
         offsetof(nxt_common_app_conf_t, working_directory),
     },
 
-    {
-        nxt_string("workers"),
-        NXT_CONF_MAP_INT32,
-        offsetof(nxt_common_app_conf_t, workers),
-    },
-
     {
         nxt_string("home"),
         NXT_CONF_MAP_CSTRZ,