Motivation:

`RedisKeyLifetime` already has "RedisKey" as a prefix so it naturally fits as a nested type.

Modifications:

- Change: `RedisKeyLifetime` to be nested in `RedisKey` and named `Lifetime`
- Rename: `RedisKeyLifetime.Lifetime` to `Duration`
- Deprecate: `RedisKeyLifetime` and the nested type `Lifetime`

Result:

The global namespace is a little less cluttered with the types falling naturally where they already are.