Kill a given Function Execution

If I have a long running Function I want to cancel, there isn't any way for me to do that today. I have to wait for the timeout I've set or restart the site.

Users would like the ability to cancel individual Functions, without impacting the whole Function App.

No files, tests, or entry points are named. Begin by locating the host/runtime flow for an individual Function execution and determine how cancellation could target one execution without affecting the rest of the Function App. Done means a given long-running Function can be cancelled without waiting for its timeout or restarting the site.

Written by the indexing model from the issue text.