Support for timeouts/cancellation

[JamesWrigley]

As discussed with @oschulz, it's often useful to be able to cancel long-running remotecall's. Ideally this would be implemented using some kind of cancellation token, with a timeout being a special case of triggering the token.

[JamesWrigley]

We now have cancellation tokens in Base 🐙 JuliaLang/julia#62429