Killswitch should move from WorkflowCanceled to WorkflowCancellation

Current situation

The WorkflowCanceled is used by the killswitch service to notify of a user-initiated cancellation. This is misleading, and add undue complexity.

Desired outcome

The killswitch service should use the WorkflowCancellation message, and allow for an associated comment. While we are at it, that service could use a dryRun query parameter too.