Removed a warning that dates back to 0.4 which emits when a same-named
relationship is placed on two mappers that inherits via joined or
single table inheritance.   The warning does not apply to the
current unit of work implementation.

Change-Id: If528ec3a2f4dc60712d9044fd1ec6c4dfbf0eadb
Fixes: #3749

Fixes: #3512
Co-Authored-By: Mike Bayer <mike_mp@zzzcomputing.com>
Change-Id: Ibd126c50eda621e2f4120ee378f7313af2d7ec3c
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/193

is random; therefore it may be called against the subclass mapper first, so
need to check .concrete on both sides, references #3630

a base class and a concrete-inherited subclass would raise an error
if those relationships were set up using "backref", while setting up the
identical configuration using relationship() instead with the conflicting
names would succeed, as is allowed in the case of a concrete mapping.
fixes #3630

Includes removal of adapt_like_to_iterable() as well
as _set_iterable(), uses __slots__ for collectionadapter,
does much less duck typing of collections.

primary key before emitting an INSERT, and merges distinct objects with
duplicate primary keys together as they are encountered, which is
essentially semi-deterministic at best.   This behavior
matches what happens already with persistent objects.
fixes #3601

relationship flag; this flag *does* have an effect when the baked
lazy loader plugin has been invoked.  clarify the intent of this
flag as an "opt out" but only has an effect when the baked system
is loaded anyway.  fixes #3572

all DBs now support ON UPDATE CASCADE other than Oracle, there's no need
to try to reimplement more functionality here and users should be encouraged
to forego natural PKs unless their target platform supports them.
references #2666

exprssed -> expressed