Add a changed/updated timestamp to message entities

I think this is worth adding. Other entities that are not revisioned still have a changed field, like comments.

+1 for adding this.

The MR does add the field successfully, however I think we need to add a few things.

1. Add an update hook that copies all of the existing message entities created timestamps to the new changed field. Currently they are all null.
2. Editing a message entity on the entity edit page message/%/edit does not update the changed date.

Thanks, The update hook worked for me.
My commit gets the changed timestamp to update when the entity is saved. It appears to be working as expected in my local environment.