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).

test/test_python_keepalive.py

@@ -37,7 +37,7 @@ def application(environ, start_response):
             "applications": {
                 "app": {
                     "type": "python",
-                    "workers": 1,
+                    "processes": { "spare": 0 },
                     "path": self.testdir + '/' + name,
                     "module": "wsgi"
                 }