update titan to include tikv/titan#169 for tikv-4.x

include the following changes
```
81814ec 2020-06-04 zbk602423539@gmail.. Fix GC may delete a already deleted blob file (#168) (#169)
80657c0 2020-06-04 zbk602423539@gmail.. Fix wrong assert delta < 0 for cocurrent compaction while flush (#172) (#176)
0db7976 2020-06-04 sre-bot@pingcap.com  Fix wrong live data size when encounter rewrite failure (#161) (#175)
c41f2a2 2020-06-04 sre-bot@pingcap.com  feat: Titan should return user value to compaction filter #163 (#164) (#174)
```
Signed-off-by: sre-bot <sre-bot@pingcap.com>