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.
Minor cleanup to simplify the tree-walk code, simplify function naming,
too.
Merge pull request #951 from wiredtiger/new-split-fast-delete-2

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