Improve detection of websocket drained status.

Will now work when you lose and regain network. Also removes the
unnecessary InitialMessageRetriever.

libsignal/service/src/main/java/org/whispersystems/signalservice/api/SignalServiceMessagePipe.java

@@ -132,8 +132,10 @@ public class SignalServiceMessagePipe {
    * {@link Optional#absent()} when an empty response is hit, which indicates the websocket is
    * empty.
    *
-   * Important: The empty response will only be hit once for each instance of {@link SignalServiceMessagePipe}.
-   * That means subsequent calls will block until an envelope is available.
+   * Important: The empty response will only be hit once for each connection. That means if you get
+   * an empty response and call readOrEmpty() again on the same instance, you will not get an empty
+   * response, and instead will block until you get an actual message. This will, however, reset if
+   * connection breaks (if, for instance, you lose and regain network).
    */
   public Optional<SignalServiceEnvelope> readOrEmpty(long timeout, TimeUnit unit, MessagePipeCallback callback)
       throws TimeoutException, IOException, InvalidVersionException

libsignal/service/src/main/java/org/whispersystems/signalservice/internal/websocket/WebSocketConnection.java

@@ -150,6 +150,8 @@ public class WebSocketConnection extends WebSocketListener {
       keepAliveSender.shutdown();
       keepAliveSender = null;
     }
+
+    notifyAll();
   }
 
   public synchronized WebSocketRequestMessage readRequest(long timeoutMillis)