reorganize mapper compile/teardown under registry

reorganize mapper compile/teardown under registry

Mapper "configuration", which occurs within the
:func:`_orm.configure_mappers` function, is now organized to be on a
per-registry basis. This allows for example the mappers within a certain
declarative base to be configured, but not those of another base that is
also present in memory. The goal is to provide a means of reducing
application startup time by only running the "configure" process for sets
of mappers that are needed. This also adds the
:meth:`_orm.registry.configure` method that will run configure for the
mappers local in a particular registry only.

Fixes: #5897
Change-Id: I14bd96982d6d46e241bd6baa2cf97471d21e7caa