fix rocksdb branch and update rocksdb (#462)

Fix rust-rocksdb/master should point to rocksdb/6.4.tikv. Also update rocksdb with the change:
```
979a985cf 2020-03-25 yiwu@pingcap.com     Add counter in perf_context to time cipher time (#158)
```
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
 [submodule "titan"]
 	path = librocksdb_sys/libtitan_sys/titan
 	url = https://github.com/tikv/titan.git

rocksdb @ 979a985c

-Subproject commit aa67f5c852a4104a7ce50898aa9d33393d66aecc
+Subproject commit 979a985cfc5de5a600046b19d76c81b0ee39b4bf