update rocksdb and titan to tikv-5.0-rc branch

Signed-off-by: Yi Wu <yiwu@pingcap.com>

.gitmodules

 [submodule "rocksdb"]
 	path = librocksdb_sys/rocksdb
 	url = https://github.com/tikv/rocksdb.git
-	branch = 6.4.tikv
+	branch = tikv-5.0-rc
 
 [submodule "titan"]
 	path = librocksdb_sys/libtitan_sys/titan
 	url = https://github.com/tikv/titan.git
-	branch = master
+	branch = tikv-5.0-rc
 
 [submodule "rocksdb-cloud"]
 	path = librocksdb_sys/librocksdb_cloud_sys/rocksdb-cloud