Removed the warnings that are emitted when the LRU caches employed
by the mapper as well as loader srtategies reach their threshold; the
purpose of this warning was at first a guard against excess cache keys
being generated but became basically a check on the "creating many
engines" antipattern.   While this is still an antipattern, the presense
of test suites which both create an engine per test as well as raise
on all warnings will be an inconvenience; it should not be critical
that such test suites change their architecture just for this warning
(though engine-per-test suite is always better).

Change-Id: I41ef8cd642d05a845f53119b196440f9d7879cd9
Fixes: #4071

Change-Id: Icc742bbeecdb7448ce84caccd63e086af16e81c1
Fixes: #4026

Change-Id: Ied786e8f9ad78f524be03a382d002dada7dd218b

Fixed bug involving JSON NULL evaluation logic added in 1.1 as part
of :ticket:`3514` where the logic would not accommodate ORM
mapped attributes named differently from the :class:`.Column`
that was mapped.

Change-Id: I1848afcfb63ad7f074f315d8d3097666069b42be
Fixes: #4031

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

a with_polymorphic, regardless of inheritance type, represents
multiple classes.  A subclass that wants to joinedload with innerjoin=True
needs to be demoted to an outerjoin because the parent entity rows
might not be of that type.   Looks more intuitive with a joined
inheritance load, but applies just as well to single or concrete.

Change-Id: I4d3d76106ae20032269f8848aad70a8e2f9422f9
Fixes: #3988

Fixed a (extremely old) bug in cascade_mappers where the
first cascade we do is against the "self" mapper, and not the
one that actually corresponds to the state given. These are
different in the case where we start with a relationship
to a class, and the instance is of a subclass, which itself
can have relationships that aren't on the base mapper.

A pretty severe bug that somehow has avoided the radar
since the beginning.

Change-Id: I512956b9757b07e06f3ca1ccb507a33fb10bed31
Fixes: #3986

The ``lazy="select"`` loader strategy now makes used of the
:class:`.BakedQuery` query caching system in all cases.  This
removes most overhead of generating a :class:`.Query` object and
running it into a :func:`.select` and then string SQL statement from
the process of lazy-loading related collections and objects.  The
"baked" lazy loader has also been improved such that it can now
cache in most cases where query load options are used.

Change-Id: Ic96792fffaa045ae9aa0a4657d6d29235d3efb85
Fixes: #3954

Added warnings to the LRU "compiled cache" used by the :class:`.Mapper`
(and ultimately will be for other ORM-based LRU caches) such that
when the cache starts hitting its size limits, the application will
emit a warning that this is a performance-degrading situation that
may require attention.   The LRU caches can reach their size limits
primarily if an application is making use of an unbounded number
of :class:`.Engine` objects, which is an antipattern.  Otherwise,
this may suggest an issue that should be brought to the SQLAlchemy
developer's attention.

Additionally, adjusted the test_memusage algorithm again as the
previous one could still allow a growing memory size to be missed.

Change-Id: I020d1ceafb7a08f6addfa990a1e7acd09f933240