In most cases when working with a ConversationItem we already have a
write transaction, but sometimes we don't. In this case, we know you
can only be blocked in a conversation if there's history in it (at
least as far as the local device knows), and so we can ignore any
threads that don't exist.
Creates a special data source enum for ConversationAvatarView data
providers. Before, ContactCell and ConversationAvatarView used the data
source, neither applying to each perfectly
