post-load of joined-inh subtables

[Mike Bayer]

using the post_load architecture of #3944, add post_with_polymorphic argument to mapper. Subclasses will be loaded using a query like query(Subclass).options(defer(everything on superclass), noload(everything on superclass)), as opposed to a custom SELECT statement the way the lazy load works, so that relationship loaders and other loader options on the subclass take effect normally.

[Mike Bayer]

selectin polymorphic loading

Added a new style of mapper-level inheritance loading "polymorphic selectin". This style of loading emits queries for each subclass in an inheritance hierarchy subsequent to the load of the base object type, using IN to specify the desired primary key values.

Fixes: #3948

Change-Id: I59e071c6142354a3f95730046e3dcdfc0e2c4de5

→ <<cset bb6a1f690d4a>>