- New event hook, MapperEvents.after_configured().

  - New event hook, MapperEvents.after_configured().
    Called after a configure() step has completed and
    mappers were in fact affected.   Theoretically this
    event is called once per application, unless new mappings
    are constructed after existing ones have been used
    already.

  - New declarative features:
        - __declare_last__() method, establishes an event
        listener for the class method that will be called
        when mappers are completed with the final "configure"
        step.
        - __abstract__ flag.   The class will not be mapped
        at all when this flag is present on the class.
        - New helper classes ConcreteBase, AbstractConcreteBase.
        Allow concrete mappings using declarative which automatically
        set up the "polymorphic_union" when the "configure"
        mapper step is invoked.
        - The mapper itself has semi-private methods that allow
        the "with_polymorphic" selectable to be assigned
        to the mapper after it has already been configured.
        [ticket:2239]