A core RSF interface specifying a "strategy" for saving and restoring
segments of a request-scope bean model. In general each strategy will be
separately configured in terms of i) which bean paths it acts on, and
ii) which target storage the state will be preserved in (generally
a TokenStateHolder). These strategies are managed by an app-global
StatePreservationManager author: Antranig Basman (antranig@caret.cam.ac.uk)