Support for timeouts/cancellation

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.

Start by reading the existing remotecall implementation and the discussion referenced in this issue. Define how cancellation tokens should behave for long-running calls, including timeout-triggered cancellation, and use that behavior as the completion criteria.

索引モデルが issue の本文から書いたものです。