vinyl: cleanup usage of tuple comparison functions

 - Use vy_tuple_compare() instead of vy_stmt_compare() in places where
   we know that both arguments are tuples (type != SELECT).

 - Use vy_tuple_compare() instead of tuple_compare() in the write
   iterator to assure that none of its arguments happens to be a key.

 - Use vy_stmt_compare() instead of vy_tuple_compare_with_key() in the
   read iterator when comparing the resulting statement to the search
   key, because the search key may be a tuple.