Proper cancellation for perform_action

[mxgrey]

Currently when a task cancellation happens while a perform_action is going on, RMF will immediately trigger the finished callback which does not give the underyling perform_action implementation an opportunity to respond to the cancellation before new commands get issued.

Ideally the underlying perform_action implementation should be able to block further execution of any commands until it is able to finish any cleanup behavior that it needs to do. We should endeavor to provide an API that lets users set a cancellation callback for their action, and the perform_action event will not trigger its finished callback until the cancellation callback has finished running.