[Code Issue] Unnecessary assignment?

[microwish]

Looks like sometimes this assignment env->kvs = envs; in the body of Environment::Init method in env.h (https://github.com/dmlc/ps-lite/blob/db6189746c2acc5e6c3611df27f38b15029a7797/include/ps/internal/env.h#L36) is not necessary.

Assume we don't expect to re-assign new environment variables once we already assigned non-empty values, it might be better if we first check whetherEnvironment::kvs is empty, if it's empty, we can do this assignment, otherwise, we won't.

If I'm on the track, would like to make a PR.

Thanks.