Churn: Rewrote churn mechanism, now using home-built

Instead of the Lifecycle feature in INET, use own solution. INET's code will throw an error if an UDP message is received by an offline host, which will eventually happen to us… So this is a first try of an homemade online/offline-mechanism that simply discards UDP packets when offline.
Removed INET's Lifecycle functionality for churn controlling

Because offline nodes throw errors when they receive UDP packets (WTF?) we can't use this functionality. Changed it to self-made one that simply discards all packets that are received when offline. Side effect: ChurnController looks way nicer and easier now …

* NEW: Extend goOnline()/goOffline() for all necessary initializations etc.
  * start/stop/crashNode() aren't used for that any more (for this class and subclasses!)
* Removed nasty bug in SimpleNode that deleted messages which were needed elsewhere
gitignore: Ignore user-supplied makefile fragments