Switch to a more heavily TOFU model for identity keys.

1) There is no longer a concept of "verified" or "unverified."
   Only "what we saw last time" and "different from last time."

2) Let's eliminate "verify session," since we're all about
   identity keys now.

3) Mark manually processed key exchanges as processed.

src/org/thoughtcrime/securesms/service/SmsReceiver.java

@@ -108,7 +108,7 @@ public class SmsReceiver {
 
         if (processor.isStale(keyExchangeMessage)) {
           message.setStale(true);
-        } else if (!processor.hasCompletedSession() || processor.hasSameSessionIdentity(keyExchangeMessage)) {
+        } else if (processor.isTrusted(keyExchangeMessage)) {
           message.setProcessed(true);
 
           Pair<Long, Long> messageAndThreadId = storeStandardMessage(masterSecret, message);