Better abstraction for database `Seek`

[infrmtcs]

However, this can be done much better, because the current abstraction NewIterator is pretty bad. It doesn't provide a lot features that are provided by pebble:

• Missing Last(), which prevent reverse iteration
• Customized range instead of by prefix only, like (key, 0) -> (key, height - 1), which is what we need in this case.
• SeekLT, which is exactly what we need here instead of SeekGE used by it.Seek. This is why we have to check if seekedHeight < height ... else if seekedHeight == height ...

Originally posted by @infrmtcs in #3253 (comment)