Fix the problem where a truncated/deleted page might split.

Fix the problem where a truncated/deleted page might split (meaning there's no single place the truncate abort can go to find the list of WT_UPDATE structures).  The primary change is to replace the WT_REF.txnid field with the WT_REF.page_del field.  The new field holds both the transaction ID, and, if necessary, an array of WT_UPDATE structures so abort can find them even after the page splits.

Reference #948.