Finalize plugin initialization on demand.
    
    This patch changes the way plugins are initialized. Instead of
    finalizing all the plugins at once, plugins are finalized only after
    they are accessed (currently applies to Command, Object and
    Attribute subclasses, only in CLI by default).
    
    This change provides significant performance boost, as only the
    plugins that are actually used are finalized.
    
    ticket 1336