fully integrate DefaultsManager with the session

[ichthyo]

There is a DefsManager instance integrated into the session, such as to be accessible on the public API. This "defaults manager" is just backed by a system of dummy config rules for the time being. But it should be hooked up with a real query subsystem and especially it should be integrated into the session implementation as a first-class (internal) service, meaning

• it should participate in the session saving/loading
• it should be subject to session's transactional behaviour (i.e. atomic switch to a new instance)