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/validationinterface.h

@@ -55,6 +55,11 @@ void SyncWithValidationInterfaceQueue();
 
 class CValidationInterface {
 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.
+     */
+    ~CValidationInterface() = default;
     /**
      * Notifies listeners of updated block chain tip
      *