Always cache the last key returned for use in scans.  Otherwise, we can do huge amounts of unnecessary work in __cursor_row_slot_return when scanning some pages.  This change speeds up test_backup03 by over 20% (by speeding up "wt dump").
Merge branch 'develop' into scan-opt
minor cleanup, only assign to kb.{data,size} once.
Merge pull request #705 from wiredtiger/scan-opt

Always cache the last key returned for use in scans.