session: use vpp to switch io events for ct sessions

Instead of allocating pairs of message queues per cut-thru session and
having the applications map them, this uses vpp as an io event message
switch.

Change-Id: I51db1c7564df479a7d1a3288342394251fd188bb
Signed-off-by: Florin Coras <fcoras@cisco.com>

test/test_vcl.py

@@ -259,8 +259,8 @@ class LDPCutThruTestCase(VCLTestCase):
                                               self.server_port]
 
     def tearDown(self):
+        self.logger.debug(self.vapi.cli("show session verbose 2"))
         self.cut_thru_tear_down()
-
         super(LDPCutThruTestCase, self).tearDown()
 
     @unittest.skipUnless(running_extended_tests, "part of extended tests")