improve state caching

[Dave Cheney]

As discussed during the July sprint, improving the caching of state objects, thereby reducing the number of error paths to be checked would be highly beneficial.

This issue is a placeholder for that discussion.

[Gustavo Niemeyer (niemeyer)]

The discussion, IIRC, evolved towards something resembling this:

- Have Refresh methods on all resources that may change

- Take the error result out of accessor methods that lookup cached information

- Have setter methods that mutate the specific field, and change the cached value

- Optionally update the cached state on mutations (even more if we're forced to reload it)

- Change watchers so they always return a new value rather than a cached one

That last step depends on lifecycle to handle removals properly.

[Roger Peppe (rogpeppe)]

That fits well with my recollection too.