for setting up baked lazy loaders would interfere with other
loader strategies that rely on lazy loading as a fallback, e.g.
joined and subquery eager loaders, leading to ``IndexError``
exceptions at mapper configuration time.
fixes #3612

directly or within lazy loads, didn't consider the mapper's "get clause"
as part of the cache key, causing bound parameter mismatches if the
clause got re-generated.  This clause is cached by mappers
on the fly but in highly concurrent scenarios may be generated more
than once when first accessed.
fixes #3597

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

Thanks to Mike Bayer for suggesting a simpler refactoring.

Also add a couple of missing tests.

- changelog / version note finishing

- alter the approach so that the initial callable is working just like add_criteria/with_criteria

to clean up automatically.  references #3407

simple but unusual system allows for a dramatic savings in Python
overhead for the construction and processing of orm :class:`.Query`
objects, from query construction up through rendering of a string
SQL statement.
fixes #3054