Use read receipt envelope to infer startExpirationTime

Avoids display of phantom messages that are only received and marked
read locally long after they have expired on another linked device.

js/background.js

@@ -247,10 +247,15 @@
     }
 
     function onReadReceipt(ev) {
-        var timestamp = ev.timestamp.toNumber();
-        var sender    = ev.sender;
+        var read_at   = ev.timestamp;
+        var timestamp = ev.read.timestamp;
+        var sender    = ev.read.sender;
         console.log('read receipt ', sender, timestamp);
-        Whisper.ReadReceipts.add({sender: sender, timestamp: timestamp});
+        Whisper.ReadReceipts.add({
+            sender    : sender,
+            timestamp : timestamp,
+            read_at   : read_at
+        });
     }
 
     // lazy hack