Introducing app joint to accurate app release.

For accurate app descriptor release, it is required to count the number of
use counts.  Use count increased when:
- app linked to configuration app queue;
- socket conf stores pointer to app;
- request for start app process posted to router service thread;

Application port has pointer to app, but it does not increase use count
to avoid use count loop.

Timer needs a pointer to nxt_timer_t which is stored in engine timers tree.
nxt_timer_t now resides in nxt_app_joint_t and does not lock the application.

Start process port RPC handlers is also linked to nxt_app_joint_t.

App joint (nxt_app_joint_t) is a 'weak pointer':
- single threaded;
- use countable;
- store pointer to nxt_app_t (which can be NULL);

nxt_app_t has pointer to nxt_app_joint_t and update its pointer to app.

test/test_python_application.py

@@ -195,8 +195,8 @@ class TestUnitPythonApplication(unit.TestUnitApplicationPython):
 
         self.assertEqual(resp, {}, 'reconfigure 2 keep-alive 3')
 
-    @unittest.expectedFailure
     def test_python_keepalive_reconfigure_3(self):
+        self.skip_alerts.append(r'sendmsg.+failed')
         self.load('empty')
 
         (resp, sock) = self.http(b"""GET / HTTP/1.1