Change query._identity_lookup into a normal instance method

Change query._identity_lookup into a normal instance method

Fixed regression in 1.2.7 caused by :ticket:`4228`, which itself was fixing
a 1.2-level regression, where the ``query_cls`` callable passed to a
:class:`.Session` was assumed to be a subclass of :class:`.Query`  with
class method availability, as opposed to an arbitrary callable.    In
particular, the dogpile caching example illustrates ``query_cls`` as a
function and not a :class:`.Query` subclass.

Change-Id: I3f86fcb12a6a9a89aa308b335e75c25969bcc30e
Fixes: #4256
(cherry picked from commit 3fa38a1a2313b4644daa431d629394d6bb14497a)