refactor: add explicit this-> where possible

I have knowingly skipped some files/portions of files where this would
create merge conflicts for other open PRs.

src/controllers/plugins/api/Message.cpp

@@ -442,7 +442,7 @@ private:
             }
             else
             {
-                return visitOne<TReturn, Rest...>(
+                return this->visitOne<TReturn, Rest...>(
                     std::forward<decltype(rest)>(rest)...);
             }
         }