Summary:
- Original commit: a4fb1f8c049ee9d61a9da8cf23b64d2c7d36a33f
- Revert commit (we reverted as a quick fix to get crash tests passing): 6afe22db2e667799d8c903db61750d676bffe152

This PR includes the contents of the original commit plus two bug fixes, which are:

- In whitebox crash test, only set `--expected_values_path` for `db_stress` runs in the first half of the crash test's duration. In the second half, a fresh DB is created for each `db_stress` run, so we cannot maintain expected state across `db_stress` runs.
- Made `Exists()` return true for `UNKNOWN_SENTINEL` values. I previously had an assert in `Exists()` that value was not `UNKNOWN_SENTINEL`. But it is possible for post-crash-recovery expected values to be `UNKNOWN_SENTINEL` (i.e., if the crash happens in the middle of an update), in which case this assertion would be tripped. The effect of returning true in this case is there may be cases where a `SingleDelete` deletes no data. But if we had returned false, the effect would be calling `SingleDelete` on a key with multiple older versions, which is not supported.
Closes https://github.com/facebook/rocksdb/pull/3793

Differential Revision: D7811671

Pulled By: ajkr

fbshipit-source-id: 67e0295bfb1695ff9674837f2e05bb29c50efc30