The WT_INTL_FOREACH_BEGIN macro walks the page's index, and it could be
split out and freed from underneath us.  Change the
WT_INTL_FOREACH_BEGIN/WT_INTL_FOREACH_END pairs to protect the index
during the walk using WT_ENTER_PAGE_INDEX/WT_LEAVE_PAGE_INDEX pairs.
Reference #1246, #1211.