message: Make QuicServerWorker::add/RemoveAcceptObserver virtual
  Summary: As titled, to enable mocks for testing.
diff: @@ -439,7 +439,7 @@ class QuicServerWorker : public folly::AsyncUDPSocket::ReadCallback,
  *
  * @param observer Observer to add (implements AcceptObserver).
  */
  - void addAcceptObserver(AcceptObserver* observer) {
  + virtual void addAcceptObserver(AcceptObserver* observer) {
  observerList_.add(observer);
  }
  @@ -449,7 +449,7 @@ class QuicServerWorker : public folly::AsyncUDPSocket::ReadCallback,
  * @param observer Observer to remove.
  * @return Whether observer found and removed from list.
  */
  - bool removeAcceptObserver(AcceptObserver* observer) {
  + virtual bool removeAcceptObserver(AcceptObserver* observer) {
  return observerList_.remove(observer);
  }
  