table/iterator.go
func (itr *Iterator) seekForPrev(key []byte) {
	// TODO: Optimize this. We shouldn't have to take a Prev step.
	itr.seekFrom(key, origin)
	if !bytes.Equal(itr.Key(), key) {
		itr.prev()
	}
}
func (itr *Iterator) seekForPrev(key []byte) {
	// TODO: Optimize this. We shouldn't have to take a Prev step.
	itr.seekFrom(key, origin)
	if !bytes.Equal(itr.Key(), key) {
		itr.prev()
	}
}
// seekForPrev will reset iterator and seek to <= key.