Clear saved value in DBIter::{Next, Prev} (#10934)

Clear saved value in DBIter::{Next, Prev} (#10934)

Summary:
`DBIter::saved_value_` stores the result of any `Merge` that was performed to compute the iterator's current value. This value can be ditched whenever the iterator's position is changed, and is already cleared in `Seek`, `SeekForPrev`, `SeekToFirst`, and `SeekToLast`. With the patch, it is also cleared in `Next` and `Prev`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10934

Test Plan: `make check`

Reviewed By: akankshamahajan15

Differential Revision: D41133473

Pulled By: ltamasi

fbshipit-source-id: cf9e936f48151e64e455cc1664d6e9f4a03aa308