Lazy initialization for default Goa mux

[tchssk]

#3415 (comment)

• I tried to implement it using only sync.Mutex.
  • middlewares is changed at the first call of Handle(), so it will be nil at the second call.
• wildcards is now accessed in the mutex lock, so I reverted Use sync.Map for http.mux.wildcards #3417.

[raphael]

This looks great thank you! Reverting the change makes sense. It does mean that the reads are not protected against concurrent writes but that's probably OK given that writes should only occur during initialization and having a lock shared across all requests wouldn't be great from a performance point of view.

[tchssk]

Yes. Now Handle() is protected with a mutex, so it's impossible that concurrent access to the map inside the method.

[tchssk]

Finally it's completed. Would you please release a new version? @raphael