Support stopping tank during scope events

[bemson]

In theory, all of Panzer's navigation events should be "asynchronous" - that is, stoppable via the tank API. Of course, in reality, the navigation loop logic is centered around traversal events. For the purposes of a plugin to Salt, I need to quickly allow Panzer to support stops during the scope event.

Panzer needs a lot of love, but I'm doing what I can as time allows. Thanks for your patience!!