bugfix: properly ending any thread and not generating seg fault when stopping amf process

Issue 1: making sure itti timer thread is finished before deallocating

Same as in SMF. Once again it can be very slow to kill.

Issue 2: amf-app is not deallocating objects

I realized this when I worked on the sctp destructor; it was never called.

Issue 3: making sure sctp_receiver_thread is finished

Here it was easier since pthread is used and we can do pthread_kill.

@arora @schmidtr @phine_stefan That should solve all AMF seg faults.