Fix ExtendContext, and export until for client to check the validity of the acquired lock.

[KeiichiHirobe]

I propose 2 changes.

Fix ExtendContext to check validity time and update until.

As explained here, we need check validity time.

The client should only consider the lock re-acquired if it was able to extend the lock into the majority of instances, and within the validity time (basically the algorithm to use is very similar to the one used when acquiring the lock).

Add Until() to export until

To export until is useful for client to check the validity of the acquired lock.
Furthermore, I think there is a potential bug in current implementation of ValidContext/Valid. I'm going to create an issue about that soon.

[hjr265]

@KeiichiHirobe Thank you for the pull request. Sorry to take a while to get to this.