Introduce registration-time ID for detecting stale sessions.

1) At registration time, a client generates a random ID and
   transmits to the the server.

2) The server provides that registration ID to any client
   that requests a prekey.

3) Clients include that registration ID in any
   PreKeyWhisperMessage.

4) Clients include that registration ID in their sendMessage
   API call to the server.

5) The server verifies that the registration ID included in
   an API call is the same as the current registration ID
   for the destination device.  Otherwise, it notifies the
   sender that their session is stale.

src/org/thoughtcrime/securesms/util/DirectoryHelper.java

@@ -1,7 +1,6 @@
 package org.thoughtcrime.securesms.util;
 
 import android.content.Context;
-import android.util.Log;
 
 import org.thoughtcrime.securesms.push.PushServiceSocketFactory;
 import org.whispersystems.textsecure.directory.Directory;

src/org/thoughtcrime/securesms/util/TextSecurePreferences.java

@@ -43,6 +43,16 @@ public class TextSecurePreferences {
   private static final String DIRECTORY_FRESH_TIME_PREF        = "pref_directory_refresh_time";
   private static final String IN_THREAD_NOTIFICATION_PREF      = "pref_key_inthread_notifications";
 
+  private static final String LOCAL_REGISTRATION_ID_PREF       = "pref_local_registration_id";
+
+  public static int getLocalRegistrationId(Context context) {
+    return getIntegerPreference(context, LOCAL_REGISTRATION_ID_PREF, 0);
+  }
+
+  public static void setLocalRegistrationId(Context context, int registrationId) {
+    setIntegerPrefrence(context, LOCAL_REGISTRATION_ID_PREF, registrationId);
+  }
+
   public static boolean isInThreadNotifications(Context context) {
     return getBooleanPreference(context, IN_THREAD_NOTIFICATION_PREF, true);
   }