I think there is a potential bug in current implementation of ValidContext/Valid.
When ValidContext/Valid returns true, there is a possibility that the lock is expired already.
For example, case N=5, we have 3 seconds before lock expires, even if 4 nodes responds soon(~100ms) , it is possibility that the last node needs 10 seconds to respond, and if that happens, ValidContext/Valid returns true, but lock is expired.

As far as I read https://redis.io/topics/distlock, I understand we should trust only the value of until.

If the lock was acquired, its validity time is considered to be the initial validity time minus the time elapsed, as computed in step 3.

So, if #71 merged, It may be a good idea to make ValidContext/Valid deprecated and client can check by calling Until or change implementation like below to keep backward compatibility.

func (m *Mutex) Valid() (bool, error) {
	return time.Now().Before(until), nil
}