Filter terminated calls

https://bugzilla.mozilla.org/show_bug.cgi?id=1058706
Big +1 to using constants everywhere. Something like `CallState.TERMINATED` in CallState.js (or whatever) so it can be easily included in other files seems decent. (we're using `"terminated` 20 times on this diff &mdash; 3 times with `"terminated:{reason}"`).

Also, something similar for `messageType` values. It looke like we have ".messageType" at least 7 times through this screen (didn't try grepping yet).

Not sure if we want to further that with constants for `message.reason` constants (only instance I see below for reason is "timeout").

Proposal:
CallStates.js
```js
const ALERTING = "alerting";
const CONNECTED = "connected";
const HALF_INITIATED = "half-initiated";
const TERMINATED = "terminated";

module.exports = {
  ALERTING: ALERTING,
  CONNECTED: CONNECTED,
  HALF_INITIATED: HALF_INITIATED,
  TERMINATED: TERMINATED
};
```

MessageTypes.js
```js
const PROGRESS = "progress";

module.exports = {
  PROGESS: PROGESS
};
```

MessageReasons.js
```js
const TIMEOUT = "timeout";

module.exports = {
  TIMEOUT: TIMEOUT
};
```

Or if that is all too many files, perhaps a single Constants.js file:
```js
module.exports = {
  CallStates: {
    ALERTING: "alerting"
  },
  MessageTypes: {
    PROGRESS: "progress"
  },
  MessageReasons: {
    TIMEOUT: "timeout"
  }
};
```

Then you can just require whatever constants you may need:
```js
var CallStates = require("./CallStates");
var MessageReasons = require("./MessageReasons");
self.broadcastState(session.callId, CallStates.TERMINATED + ":" + MessageReasons.TIMEOUT);

// or

var Constants = require("./Constants");
self.broadcastState(session.callId, Constants.CallStates.TERMINATED + ":" + Constants.MessageReasons.TIMEOUT);
```