Add a missing virtual destructor to CollectionChangeCallback

This resulted in the destructor for the callback object passed to
`add_notification_callback()` not being called, potentially resulting in memory
leaks when removing notification callbacks.

src/collection_notifications.hpp

@@ -126,6 +126,7 @@ class CollectionChangeCallback {
 
 private:
     struct Base {
+        virtual ~Base() {}
         virtual void before(CollectionChangeSet const&)=0;
         virtual void after(CollectionChangeSet const&)=0;
         virtual void error(std::exception_ptr)=0;