Support I/O cancellation

[lukechampine]

It should be possible to cancel PseudoFile operations like Read and Write. This would imply support for cancellation at the proto level as well.

I think the best way to accomplish this would be SetDeadline methods on PseudoFile and Session. These would translate directly to SetDeadline calls on the underlying net.Conn. If a timeout occurs, the caller should be able to determine this by unwrapping the resulting error.

Adding context.Context arguments to every method is also an option, but I've never liked this pattern. It pollutes the API and drags in non-cancellation baggage that is almost never used.

[jkawamoto]

It's not easy to implement graceful shutdown without context-based cancellation, isn't it?

On the other hand, from this discussion, we should use a longer timeout such as a couple of minutes: https://discordapp.com/channels/341359001797132308/341362900117487636/740999604169998436

Then, SetDeadline-based cancellation would block a goroutine for a long time.