WT-3373 Access violation due to a bug in internal page splitting (#3478)

When acquiring a lock on our parent internal page, we use the WT_REF.home
field to reference our parent page.  As a child of the parent page, we
prevent its eviction, but that's a weak guarantee. If the parent page
splits, and our WT_REF were to move with the split, the WT_REF.home field
might change underneath us and we could race, and end up attempting to
access an evicted page. Set the session page-index generation so if the
parent splits, it still can't be evicted.