[SPARK-29879][CORE]Support unbounded waiting for RPC reply

[wenxuanguan]

What changes were proposed in this pull request?

Support unbounded waiting for RPC reply with the call RpcEndpointRef.askSync[T: ClassTag](message: Any, timeout: RpcTimeout)

Why are the changes needed?

At present, we hard code long duration to represent never timeout as below

timeout = new RpcTimeout(365.days, "barrierTimeout")

with the support of unbounded waiting, we can do as below

timeout = new RpcTimeout(Duration.Inf, "barrierTimeout")

Does this PR introduce any user-facing change?

No

How was this patch tested?

N/A

[AmplabJenkins]

Can one of the admins verify this patch?

[jiangxb1987]

Do you really have a training job that runs over a year?

[wenxuanguan]

@jiangxb1987 Thanks for your reply.

Do you really have a training job that runs over a year?

No, but what I try to do is a way to not set a specific timeout value with RpcEndpointRef.askSync(), in the situation where never timeout until replied or interrupted by other event.
Such as BarrierTaskContext.barrier() set a long timeout value which would not be reached, and a SparkException will be thrown by BarrierCoordinator on timeout.

[jiangxb1987]

sorry I'm not convinced this change is necessary.

[wenxuanguan]

@jiangxb1987 ok, and I will close the pr. Thanks for your review.