Simplify background parsing/typechecking

This avoids a lot of needless reparses. All these reparses make the navigation extension nervous. Prior to this change, the nav bar would typically change path three times when switching active document.
It seems Parse() is designed to be long running, and it won't lock the UI. So there's no need to do this trickery with requesting reparses.

Can anyone see a reason to not do this?

Tested on Mac and Windows.