shorten the parent page lock duration when splitting

@michaelcahill: I moved the child page discard and the building of the split page's replacement WT_REF structures outside of the parent page lock in the split code.  I doubt there are many workloads this will help, but I wanted to make the change while I still remembered how it all worked.   It's slightly  more complicated this way, but it means almost no heavy-weight operations while holding the parent locked.