Support multiple env instance

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We have multiple disk on server and several rocksdb instances share one disk. It better to have one env per disk for isolation.
Currently rocksdb support global env by Env::Default() and user can't create env because env constructor is private.

It better to add Env::NewEnv so user can create new default env.

### Expected behavior
user can create multiple env instance by Env::NewEnv()
### Actual behavior
currently only support env::Default()
### Steps to reproduce the behavior