[tikv-3.x] Update to latest Titan and fix submodule url (#433)

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.

.gitmodules

 [submodule "rocksdb"]
 	path = librocksdb_sys/rocksdb
-	url = https://github.com/pingcap/rocksdb.git
+	url = https://github.com/tikv/rocksdb.git
 	branch = tikv-3.x
 [submodule "titan"]
 	path = librocksdb_sys/libtitan_sys/titan
-	url = https://github.com/pingcap/titan.git
+	url = https://github.com/tikv/titan.git
 	branch = tikv-3.x

README.md

@@ -40,7 +40,7 @@ $ cargo build
 
 ```rust
 [dependencies.rocksdb]
-git = "https://github.com/pingcap/rust-rocksdb.git"
+git = "https://github.com/tikv/rust-rocksdb.git"
 ```
 
 ###### Code

titan @ b623e6be

-Subproject commit d0af5c7e1ec193787de34f418e10455754972e73
+Subproject commit b623e6be36d3d1a59bc9e73ca65eed1693abe0ef