Optimize session history.

When MaxScale is used from PHP the sessions are kept alive by PHP and hence are quite long, which causes the session history to grow quite big. So big that it is often taken as a sign of MaxScale leaking memory.

Earlier MaxScale closed the connection when the history grew larger than a specified limit. With recent changes, the history is turned off, which means that for that particular session only those slaves that have been used, can be used. If one of those goes down, then there is one slave less and another cannot be used.

If there is duplication in the session history of a form that could be folded, then it might be possible to make the growth of the session history slower, perhaps so that it after a certain point no longer grows.