Adds a new variant to the "isnew" state within entity loading
for isnew=False, but the load path is new.  This is to address
the use case of an entity appearing in multiple places in
the row in a more generalized way than the fixes in [ticket:3431],
[ticket:3811] in that loading.py will be able to tell the
populator that this row is not "isnew" but is a "new" path
for the entity.   For the moment, the new information is only
being applied to the use of "quick" populators so that
simple column loads can take place on top of a deferred loader
from elsewhere in the row.

As part of this change, state.load_path() will now always
be populated with the "path" that was in effect when this state
was originally loaded, which for multi-path loads of the
same entity is still non-deterministic.  Ideally there'd be some
kind of "here's all the paths that loaded this state and how"
type of data structure though it's not clear if that could be
done while maintaining performance.

Fixes: #3822
Change-Id: Ib915365353dfcca09e15c24001a8581113b97d5e

Discovered during testing for [ticket:3822], the rule added
for [ticket:1495] will fail if the source object has propagated
options set up, which add elements to query._current_path.

Fixes: #3824
Change-Id: I3d96c96fee5f9b247f739d2136d18681ac61f2fe

Fix improper capture of a loop variable inside a lambda during unbaking
of subquery eager loaders, which would cause the incorrect query
to be invoked.

Fixes: #3743
Change-Id: I995110deb8ee2dae8540486729e1ae64578d28fc
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/290

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

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