Handle pubsub callbacks from a cached thread pool.

...implement some belt and suspenders dead letter handling.

...implement some belt and suspenders redis pubsub queue handling.

// FREEBIE

src/main/java/org/whispersystems/textsecuregcm/websocket/DeadLetterHandler.java

@@ -0,0 +1,39 @@
+package org.whispersystems.textsecuregcm.websocket;
+
+import com.google.protobuf.InvalidProtocolBufferException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.whispersystems.textsecuregcm.entities.MessageProtos.OutgoingMessageSignal;
+import org.whispersystems.textsecuregcm.storage.MessagesManager;
+import org.whispersystems.textsecuregcm.storage.PubSubProtos;
+
+public class DeadLetterHandler {
+
+  private final Logger logger = LoggerFactory.getLogger(DeadLetterHandler.class);
+
+  private final MessagesManager messagesManager;
+
+  public DeadLetterHandler(MessagesManager messagesManager) {
+    this.messagesManager = messagesManager;
+  }
+
+  public void handle(byte[] channel, PubSubProtos.PubSubMessage pubSubMessage) {
+    try {
+      WebsocketAddress address = new WebsocketAddress(new String(channel));
+
+      logger.warn("Handling dead letter to: " + address);
+
+      switch (pubSubMessage.getType().getNumber()) {
+        case PubSubProtos.PubSubMessage.Type.DELIVER_VALUE:
+          OutgoingMessageSignal message = OutgoingMessageSignal.parseFrom(pubSubMessage.getContent());
+          messagesManager.insert(address.getNumber(), address.getDeviceId(), message);
+          break;
+      }
+    } catch (InvalidProtocolBufferException e) {
+      logger.warn("Bad pubsub message", e);
+    } catch (InvalidWebsocketAddressException e) {
+      logger.warn("Invalid websocket address", e);
+    }
+  }
+
+}