Polish interfaces around PeerLogicValidation

* Make PeerLogicValidation final to prevent deriving from it [1]
* Prevent deletions of NetEventsInterface and CValidationInterface
  objects via a base class pointer

[1] silences the following compiler warning (from Clang 7.0.0):

/usr/include/c++/v1/memory:2285:5: error: delete called on non-final 'PeerLogicValidation' that has
      virtual functions but non-virtual destructor [-Werror,-Wdelete-non-virtual-dtor]
    delete __ptr;
    ^
/usr/include/c++/v1/memory:2598:7: note: in instantiation of member function
      'std::__1::default_delete<PeerLogicValidation>::operator()' requested here
      __ptr_.second()(__tmp);
      ^
init.cpp:201:15: note: in instantiation of member function 'std::__1::unique_ptr<PeerLogicValidation,
      std::__1::default_delete<PeerLogicValidation> >::reset' requested here
    peerLogic.reset();
                  ^

src/net.h

@@ -469,6 +469,13 @@ public:
     virtual bool SendMessages(CNode* pnode, std::atomic<bool>& interrupt) = 0;
     virtual void InitializeNode(CNode* pnode) = 0;
     virtual void FinalizeNode(NodeId id, bool& update_connection_time) = 0;
+
+protected:
+    /**
+     * Protected destructor so that instances can only be deleted by derived classes.
+     * If that restriction is no longer desired, this should be made public and virtual.
+     */
+    ~NetEventsInterface() = default;
 };
 
 enum