Implement delivery receipts.

1) Support a "receipt" push message type.

2) Identify messages by timestamp.

3) Introduce a JobManager to handle the queue for network
   dependent jobs.

library/protobuf/IncomingPushMessageSignal.proto

@@ -10,6 +10,7 @@ message IncomingPushMessageSignal {
     KEY_EXCHANGE  = 2;
     PREKEY_BUNDLE = 3;
     PLAINTEXT     = 4;
+    RECEIPT       = 5;
   }
   optional Type   type         = 1;
   optional string source       = 2;