ExpireTime and PExpireTime should return times instead of durations

Expected Behavior

redis.ExpireTime and redis.PExpireTime should return redis.TimeCmd.

Current Behavior

redis.ExpireTime and redis.PExpireTime should return redis.DurationCmd.

Possible Solution

Ideally both functions (intruduced only recently in #2426) would return redis.TimeCmd to avoid the necessary conversion and possible confusion of returning a duration that actually represents an absolute unix timestamp.

This would however break the v9 API :disappointed:

Alternatively, the discrepancy should be clearly explained in the API, warning about the need for conversion.