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_conf.h

@@ -100,6 +100,7 @@ void nxt_conf_json_position(u_char *start, u_char *pos, nxt_uint_t *line,
 nxt_int_t nxt_conf_validate(nxt_conf_validation_t *vldt);
 
 void nxt_conf_get_string(nxt_conf_value_t *value, nxt_str_t *str);
+int64_t nxt_conf_get_integer(nxt_conf_value_t *value);
 
 // FIXME reimplement and reorder functions below
 nxt_uint_t nxt_conf_object_members_count(nxt_conf_value_t *value);