Implement Graceful Process Termination for Exec Function

This is a narrowing of Implement Graceful Function Termination (!460 - closed) to implement Graceful Job Termination (#298 - closed) only.

This only handles graceful process termination of sub-processes created by the Exec Function. Function cancelation (gracefully or otherwise) will happen elsewhere.

Note that the graceful exit delay value is, for now, hard coded at 30 seconds. Eventually the value will come from the client via either the RunRequest or CancelRequst APIs, or some other mechanism.

Closes Graceful Job Termination (#298 - closed)