Summary:
Before this PR, we call `now()` to get the wall time before performing point-lookup and range
scans when user-defined timestamp is enabled.

With this PR, we expand the coverage to:
- read with an older timestamp which is larger then the wall time when the process starts but potentially smaller than now()
- add coverage for `ReadOptions::iter_start_ts != nullptr`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10280

Test Plan:
```bash
make check
```

Also,
```bash
TEST_TMPDIR=/dev/shm/rocksdb make crash_test_with_ts
```

So far, we have had four successful runs of the above

In addition,
```bash
TEST_TMPDIR=/dev/shm/rocksdb make crash_test
```
Succeeded twice showing no regression.

Reviewed By: ltamasi

Differential Revision: D37539805

Pulled By: riversand963

fbshipit-source-id: f2d9887ad95245945ce17a014d55bb93f00e1cb5