Fixed race condition in ORM identity map which would cause objects
to be inappropriately removed during a load operation, causing
duplicate object identities to occur, particularly under joined eager
loading which involves deduplication of objects.  The issue is specific
to garbage collection of weak references and is observed only under the
Pypy interpreter.

Change-Id: I9f6ae3fe5b078f26146af82b15d16f3a549a9032
Fixes: #4068

Added ``KeyError`` checks to all methods within
:class:`.WeakInstanceDict` where a check for ``key in dict`` is
followed by indexed access to that key, to guard against a race against
garbage collection that under load can remove the key from the dict
after the code assumes its present, leading to very infrequent
``KeyError`` raises.

Change-Id: I881cc2899f7961d29a0549f44149a2615ae7a4ea
Fixes: #4030

Change-Id: I4e8c2aa8fe817bb2af8707410fa0201f938781de

unambiguous tracking of all object lifecycle state transitions
in terms of the :class:`.Session` itself, e.g. pending,
transient,  persistent, detached.   The state of the object
within each event is also defined.
fixes #2677
- Added a new session lifecycle state :term:`deleted`.  This new state
represents an object that has been deleted from the :term:`persistent`
state and will move to the :term:`detached` state once the transaction
is committed.  This resolves the long-standing issue that objects
which were deleted existed in a gray area between persistent and
detached.   The :attr:`.InstanceState.persistent` accessor will
**no longer** report on a deleted object as persistent; the
:attr:`.InstanceState.deleted` accessor will instead be True for
these objects, until they become detached.
- The :paramref:`.Session.weak_identity_map` parameter is deprecated.
See the new recipe at :ref:`session_referencing_behavior` for
an event-based approach to maintaining strong identity map behavior.
references #3517

where we refer to the "weak_identity_map" option, and add additional
exposition in the session documentation which refers to it.
fixes #3517

in callcounts specific to the "expiration" of objects, as in
the "auto expire" feature of :meth:`.Session.commit` and
for :meth:`.Session.expire_all`, as well as in the "cleanup" step
which occurs when object states are garbage collected.
fixes #3307

- don't do a bool on identity map since it calls __len__