Provide a mechanism for a JavaScript function to know it has run out of time

For .NET functions, there is a CancellationToken that notifies when the function has exceeded its allotted execution time. For JavaScript, we don't have such mechanism.

For comparison, Lambda has a remaining time property on the Context object (http://docs.aws.amazon.com/lambda/latest/dg/nodejs-prog-model-context.html).

Review the host's JavaScript function execution path alongside the existing .NET CancellationToken behavior. Compare the requested mechanism with Lambda's Context remaining-time property. Done means JavaScript functions can reliably detect that their allotted execution time is nearly or already exhausted.

Written by the indexing model from the issue text.