Added a stricter cancel for navigation and execution.
Added a stricter cancel for navigation and execution.

This new version does not wait to receive a callback from the cancelled action server. This is dangerous in that the next task may start while the previous task is still ending, but there isn't a huge problem with this in our current tasks. A better solution would be to wait a bit, then give up on waiting for the callback, but this is hard in the current design. Probably needs to be reimplemented as a state machine to make this cleaner.

mudrole1/strands_executive,mudrole1/strands_executive,bfalacerda/strands_executive,mudrole1/strands_executive,bfalacerda/strands_executive