Add option to turn off/on scroll to bottom in performBatchUpdates

Maybe a bool flag or an enum.
In my case I have a requirement to keep showing first new message even if they keep coming.
For now I will have to override _scrollToBottom_ and do not call super implementation in this case.
But it will be cool to have a more "legal" option.

Start by locating performBatchUpdates and the _scrollToBottom_ override mentioned in the issue. Read how batch updates currently trigger scrolling, then determine how an option could preserve the first new message while messages continue arriving. Done means callers can control the automatic scroll behavior without overriding the method.

Written by the indexing model from the issue text.