Summary:
in DBIter::SeekToLast(), key() can be called when iter is invalid and fails the following assertion:
```
./db/db_iter.h:153: virtual rocksdb::Slice rocksdb::DBIter::key() const: Assertion `valid_' failed.
```
This happens when `iterate_upper_bound` and timestamp_lb_ are set. SeekForPrev(*iterate_upper_bound_) positions the iterator on the same user key as *iterate_upper_bound_. A subsequent PrevInternal() call makes the iterator invalid just be the call to key().

This PR fixes this issue by setting updating the seek key to have max sequence number AND max timestamp when the seek key has the same user key as *iterate_upper_bound_.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11223

Test Plan: - Added a unit test that would fail the above assertion before this fix.

Reviewed By: jowlyzhang

Differential Revision: D43283600

Pulled By: cbi42

fbshipit-source-id: 0dd3999845b722584679bbc95be2664b266005ba