Retrieve conversations by ID

When you already have a set of conversationID values, retrieve their latest conversation snapshots in one call:

[[OIMManager manager] getMultipleConversation:conversationIDs
                                    onSuccess:^(NSArray<OIMConversationInfo *> *items) {
                                        [conversationStore mergeByConversationID:items ?: @[]];
                                    }
                                    onFailure:nil];

The array passed to the success callback is nullable. Treat an ID with no matching result as a missing record; do not assume that response items correspond positionally to every requested ID. The query itself does not trigger a delegate.