state: introduce automatic multi-environment filtering

For collections that contain data for multiple environments,
getCollection now returns a wrapped collection that automatically
filters by environment UUID as required. This reduces the risk of
unintended data leakage between environments.

There are some areas where an actual mgo.Collection is required. These
now use the new State.getRawCollection method.

Now that this is handled automatically, this change also removes
several TODOs regarding filtering by environment UUID.

A later branch will remove all the now-unnecessary docID calls that
currently exist in state.