Added transaction to fix threads

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -875,6 +875,9 @@ public class MessageHelper {
 
         String thread = null;
         DB db = DB.getInstance(context);
+        try {
+            db.beginTransaction();
+
             for (String ref : refs) {
                 List<EntityMessage> messages = db.message().getMessagesByMsgId(account, ref);
                 for (EntityMessage message : messages) {
@@ -884,6 +887,12 @@ public class MessageHelper {
                         db.message().setMessageThread(message.id, thread);
                 }
             }
+
+            db.setTransactionSuccessful();
+        } finally {
+            db.endTransaction();
+        }
+
         if (thread != null)
             return thread;