Summary:
As title. Previous assumption was that the underlying lib can always return
a shared_ptr<Env>. This is too strong. Therefore, we use Env::LoadEnv to relax
it.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6196

Test Plan: make check

Differential Revision: D19133199

Pulled By: riversand963

fbshipit-source-id: c83a0c02a42610d077054f2de1acfc45126b3a75