Reset all app data on WorkerChannel during env reload

[ejizba]

For placeholders, we're currently relying on the fact that the default dummy app is not a Node.js app (I think it's a csx app). Theoretically if the dummy app was a Node.js app (especially a v4 app), some of its data (like hooks, registered functions, etc.) could bleed over into the real customer's app after a func env reload.

We should consider a clean reset of all app-related data just to be safe. Also if we reset all app-related data, we don't have to worry about forgetting to reset a random boolean like in #669.