Fixed bug where an :func:`.undefer_group` option would not be recognized
if it extended from a relationship that was loading using joined eager
loading.

In particular we need to double check the column both in terms of
the given "adapter" as well as without applying the "adapter"
when searching for the column in the result.

As we now avoid redoing the row processor step we also
improve on callcounts in joined eager loading.

Change-Id: I0f48766f12f7299f4626ff41a00bf1f5bfca5f3b
Fixes: #4048

Change-Id: Icc742bbeecdb7448ce84caccd63e086af16e81c1
Fixes: #4026

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

Session.refresh() is still hardcoded to legacy lockmode,
come up with a new API so that the newer argument style
works with it.

Added new argument :paramref:`.with_for_update` to the
:meth:`.Session.refresh` method.  When the :meth:`.Query.with_lockmode`
method were deprecated in favor of :meth:`.Query.with_for_update`,
the :meth:`.Session.refresh` method was never updated to reflect
the new option.

Change-Id: Ia02a653746b7024699b515451525a88d7a17d63a
Fixes: #3991

Adding a new kind of relationship loader that is
a cross between the "immediateload" and the "subquery"
eager loader, using an IN criteria to load related items
in bulk immediately after the lead query result is loaded.

Change-Id: If13713fba9b465865aef8fd50b5b6b977fe3ef7d
Fixes: #3944

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

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

handled within visit_select(); this attribute was added in the
1.0 series to accommodate the subquery wrapping behavior of
SQL Server and Oracle while also working with positional
column targeting and no longer relying upon "key fallback"
in order to target columns in such a statement.  The IBM DB2
third-party dialect also has this use case, but its implementation
is using regular expressions to rewrite the textual SELECT only
and does not make use of a "wrapped" select at this time.
The logic no longer attempts to reconcile proxy set collections as
this was not deterministic, and instead assumes that the select()
and the wrapper select() match their columns postionally,
at least for the column positions they have in common,
so it is now very simple and safe.  fixes #3657.
- as a side effect of #3657 it was also revealed that the
strategy of calling upon a ResultProxy._getter was not
correctly calling into NoSuchColumnError when an expected
column was not present, and instead returned None up to
loading.instances() to produce NoneType failures; added
a raiseerr argument to _getter() which is called when we
aren't expecting None, fixes #3658.