Implement gateway resume url handling (#2203)

src/main/java/net/dv8tion/jda/internal/requests/WebSocketClient.java

@@ -90,6 +90,7 @@ public class WebSocketClient extends WebSocketAdapter implements WebSocketListen
     protected volatile String sessionId = null;
     protected final Object readLock = new Object();
     protected Decompressor decompressor;
+    protected String resumeUrl = null;
 
     protected final ReentrantLock queueLock = new ReentrantLock();
     protected final ScheduledExecutorService executor;
@@ -345,7 +346,7 @@ protected synchronized void connect()
             throw new RejectedExecutionException("JDA is shutdown!");
         initiating = true;
 
-        String url = api.getGatewayUrl()
+        String url = (resumeUrl != null ? resumeUrl : api.getGatewayUrl())
                 + "?encoding=" + encoding.name().toLowerCase()
                 + "&v=" + JDAInfo.DISCORD_GATEWAY_VERSION;
         if (compression != Compression.NONE)
@@ -378,6 +379,7 @@ protected synchronized void connect()
         }
         catch (IOException | WebSocketException e)
         {
+            resumeUrl = null;
             api.resetGatewayUrl();
             //Completely fail here. We couldn't make the connection.
             throw new IllegalStateException(e);
@@ -738,6 +740,7 @@ protected void sendResume()
 
     protected void invalidate()
     {
+        resumeUrl = null;
         sessionId = null;
         sentAuthInfo = false;
 
@@ -926,6 +929,7 @@ protected void onDispatch(DataObject raw)
                     // otherwise the audio connection requests that are currently pending might be removed in the process
                     handlers.get("READY").handle(responseTotal, raw);
                     sessionId = content.getString("session_id");
+                    resumeUrl = content.getString("resume_gateway_url", null);
                     break;
                 case "RESUMED":
                     reconnectTimeoutS = 2;