Add baked.Result.with_post_criteria method

Add baked.Result.with_post_criteria method

Added new method :meth:`.baked.Result.with_post_criteria` to baked
query system, allowing non-SQL-modifying transformations to take place
after the query has been pulled from the cache.  Among other things,
this method can be used with :class:`.horizontal_shard.ShardedQuery`
to set the shard identifier.   :class:`.horizontal_shard.ShardedQuery`
has also been modified such that its :meth:`.ShardedQuery.get` method
interacts correctly with that of :class:`.baked.Result`.

Change-Id: I04630c683240abbb4b99f0510a1a3dcb564815b4
Fixes: #4135