Fixed a bug in tokudb.
In a NO_DUP database, if we have a cursor pointing to an element,
and then insert (overwrite) an element with the same key but larger (as per cmp func)
data, then DB_NEXT should NOT return the newly inserted element.. it should
return the element with the next larger key.

The opposite error also existed for DB_PREV.

We rewrite the flag of DB_NEXT/DB_PREV to DB_NEXT_NODUP and DB_PREV_NODUP
when there are no duplicates.

git-svn-id: file:///svn/tokudb@3155 c7de825b-a66e-492c-adef-691d508d4ae1