Bug 918970 - display existing thread when sending a sms from contact app

Thinking more about this, I think we don't cover at least ```sending```, ```error```, ```not-downloaded``` cases, but only ```sent``` and ```received```. Sorry, I haven't read all discussion on the bug, but is it done on purpose? If not, can we just collapse it to more simple check that doesn't rely on message delivery status and type? As far as I know we can have only one of these three at the same time ```sender```, ```receiver``` or ```receivers```, but let's confirm with @steveck-chung.

Steve, do you know if message can have any combination of ```sender```, ```receiver``` and ```receivers``` at the same time or they all are mutually exclusive? 

If so, we probably can do something like this:
```
function checkCandidate(message) {
  // Skip Group MMS and continue to the next message
  if (message.receivers && message.receivers.length > 1) {
    return true;
  }

  var numberToVerify = message.sender ||
     message.receiver || (message.receivers && message.receivers[0]);
  if (Utils.probablyMatches(numberToVerify, number)) {
    threadId = message.threadId || null;
    return false; // found the message, stop iterating
  }
}
```