Summary:
Since we already parse env_uri from command line and creates custom Env
accordingly, we should invoke the methods of such Envs instead of using
Env::Default().
Test Plan (on devserver):
```
$make db_bench db_stress
$./db_bench -benchmarks=fillseq
./db_stress
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5943
Differential Revision: D18018550
Pulled By: riversand963
fbshipit-source-id: 03b61329aaae0dfd914a0b902cc677f570f102e3
c0abc6bbc1