Lazy Load Value for DBIter::Prev

[rockeet]

Now DBIter::Next supports lazy load value, but DBIter::Prev does not support lazy load for first visible is kValueType.

DBIter::Prev needs to calling underlying iter->Prev to get the iter pos of first visible kValueType, this needs to backup iter->value(), thus can not realize lazy load.

ToplingZipTable can load the value by ValueID, thus we can backup the ValueID instead of value content to realize lazy load. -- If zero copy is applicable, the lazy load is not needed.