Include Session._query_cls as part of the cache key

Include Session._query_cls as part of the cache key

Fixed issue where :class:`.BakedQuery` did not include the specific query
class used by the :class:`.Session` as part of the cache key, leading to
incompatibilities when using custom query classes, in particular the
:class:`.ShardedQuery` which has some different argument signatures.

Fixes: #4328
Change-Id: I829c2a8b09c91e91c8dc8ea5476c0d7aa47028bd