Update Titan with changes:
```
b623e6b 2020-02-04 yiwu@pingcap.com     Sync titan directory when adding blob files (#136) (#141)
0ce4d66 2020-02-01 yiwu@pingcap.com     Fix GC stat not being updated after DeleteFilesInRange (#135) (#137)
```
Also fix submodule url.