Archive sessions and generate new registration id on re-register

Fixes #6703
// FREEBIE

src/org/thoughtcrime/securesms/crypto/SessionUtil.java

@@ -41,6 +41,9 @@ public class SessionUtil {
         }
       }
     }
+  }
 
+  public static void archiveAllSessions(Context context) {
+    new TextSecureSessionStore(context).archiveAllSessions();
   }
 }