Summary:
Make titandb_stress print rocksdb and titan git hash at the beginning. This is useful in schordinger test to verify which version is being tested.

Test Plan:
run titandb_stress, which prints the following at the beginning:
```
2019/10/25-05:59:22  Initializing db_stress
RocksDB version           : 6.4
RocksDB hash              : rocksdb_build_git_sha:@9ab5a5f9e70d2ea5fc9bd22a455d52a5d082fd57@
Titan hash                : titan_build_git_sha:@b497731e@
```
Signed-off-by: Yi Wu <yiwu@pingcap.com>