Rename FeaturesManager::configCollection and related variables and methods

Problem/Motivation

Core's config API supports multiple collections, with StorageInterface::DEFAULT_COLLECTION as the default. Confusingly, however, the Features codebase uses its own concept of a config collection.

Proposed resolution

Change ::configCollection and $config_collection and any related code to ::config and $config.

Remaining tasks

User interface changes

API changes

Data model changes