Fix jolting animation after sending a message

When we send the nth message, we retain the footer for the n-1th message
until the nth message is successfully sent.

1. leave the footer on the n-1th message
2. draw the nth message with the "..." footer
3. update the nth message with the "sent" footer and hide the footer for
   the n-1th message

In the normal case this all happens very quickly, which results in some
unpleasant flicker every time you send a message.

The concession here is that we'll only print the n-1th footer if the
outgoing message *fails*. So until we implement a design more like
Android's double-check for sent/delivery, the user will not be able to
distinguish between a sent and delivered message *while* their message
is outgoing.

In my opinion the lack of jank in the normal case makes the app
supercedes this use case.

// FREEBIE

Signal/src/ViewControllers/ConversationView/MessagesViewController.m

@@ -1896,7 +1896,7 @@ typedef enum : NSUInteger {
 
             // Or when the next message is *not* an outgoing sent/delivered message.
             TSOutgoingMessage *nextMessage = [self nextOutgoingMessage:indexPath];
-            if (nextMessage && nextMessage.messageState != TSOutgoingMessageStateSentToService) {
+            if (nextMessage && nextMessage.messageState == TSOutgoingMessageStateUnsent) {
                 [self updateLastDeliveredMessage:message];
                 return result;
             }