Recommend reloadable = false with Godot 3.3, and detect when `nativescript_init` is being called a second time

Many bugs and counterintuitive behaviors have been introduced in Godot 3.3 with the hot-reload feature, and many use cases that wouldn't have required reloadable = false do now. As a fix to one of the bugs, Godot may, in the future, call nativescript_init multiple times without terminating it first. Unlike gdnative_terminate, there is no nativescript_terminate callback corresponding to the function, so we'll need to be able to detect it in some other way (possibly with an AtomicBool) and perform the appropriate cleanup actions.

See also: #746
Related engine PR: godotengine/godot#49467