Define a more generic callback type that returns `unknown` instead of `void`

The package has implicit type for `get`, `all`, etc. The type of `get` is inferred as: ```ts (method) RedisStore.get(sid: string, cb?: (_err?: unknown, _data?: any) => void): Promise<void> ``` Whereas based on the implementation the `get` can be awaited and a value can be returned. A better approach would've been to use generics, but for now a simple change to `unknown` from `void` is more than acceptable.