When returning from WT_CURSOR.update, we previously called __wt_kv_return
to point the cursor at internal copies of the data.  That no longer works
because we don't have a reference to the correct WT_UPDATE structure in
that code (and it's unclear to me how to get a reference to the correct
WT_UPDATE structure).  As a workaround, ensure the cursor has a local copy
of the key/value pair.  This is probably wrong, I'm expecting to revisit
this change once I have a better understanding of issue #1070.