Implement relationship to AliasedClass; deprecate non primary mappers

Implement relationship to AliasedClass; deprecate non primary mappers

Implemented a new feature whereby the :class:`.AliasedClass` construct can
now be used as the target of a :func:`.relationship`.  This allows the
concept of "non primary mappers" to no longer be necessary, as the
:class:`.AliasedClass` is much easier to configure and automatically inherits
all the relationships of the mapped class, as well as preserves the
ability for loader options to work normally.

- introduce new name for mapped_table, "persist_selectable".  this is
the selectable that selects against the local mapper and its superclasses,
but does not include columns local only to subclasses.

- relationship gains "entity" which is the mapper or aliasedinsp.

- clarfiy name "entity" vs. "query_entity" in loader strategies.

Fixes: #4423
Fixes: #4422
Fixes: #4421
Fixes: #3348
Change-Id: Ic3609b43dc4ed115006da9ad9189e574dc0c72d9