Summary:
- Use `__unused__` everywhere
- Suppress unused warnings in Release mode
    + This currently affects non-MSVC builds (e.g. mingw64).
Closes https://github.com/facebook/rocksdb/pull/3448

Differential Revision: D6885496

Pulled By: miasantreble

fbshipit-source-id: f2f6adacec940cc3851a9eee328fafbf61aad211