expose the underlying AsyncUDPSocket in AsyncUDPServerSocket

Summary: users may need AsyncUDPSocket to send back datagram to clients.

Reviewed By: yfeldblum

Differential Revision: D18764100

fbshipit-source-id: 7ab7cce5711f47db92f43a68b94568729d5746d0

folly/io/async/AsyncUDPServerSocket.h

@@ -148,6 +148,10 @@ class AsyncUDPServerSocket : private AsyncUDPSocket::ReadCallback,
     return socket_->getNetworkSocket();
   }
 
+  const std::shared_ptr<AsyncUDPSocket>& getSocket() const {
+    return socket_;
+  }
+
   void close() {
     CHECK(socket_) << "Need to bind before closing";
     socket_->close();