Add MakeUnique (substitute for C++14 std::make_unique)

From @ryanofsky:s #10973. Thanks!

src/util.h

@@ -326,4 +326,11 @@ template <typename Callable> void TraceThread(const char* name,  Callable func)
 
 std::string CopyrightHolders(const std::string& strPrefix);
 
+//! Substitute for C++14 std::make_unique.
+template <typename T, typename... Args>
+std::unique_ptr<T> MakeUnique(Args&&... args)
+{
+    return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
+}
+
 #endif // BITCOIN_UTIL_H