make BrooklynClassLoadingContext instances serializable

clean up how we set and manage those; fix bug where serialized state of `/Users/alex/Data/cloudsoft/dev/gits/brooklyn/docs/use/guide/defining-applications/example_yaml/appserver-clustered-w-db.yaml` was 8 MB !  also rename methods and internal fields to make things more standardized, and dampen down some logging
fix hot proxying seeing old entities (below the app level)

fix bug where newer real entities preRegistered were not being picked up, in favour of older instances, so new data was not being seen at children at hot_proxies