remove class-level sticky session parameter

This cleans up a lot of stuff. Now we can treat the session as a normal feed_manager parameter. Since there is usually only one feed_manager in operation at any time, it is basically a static member. And if it isn't, that's fine too: anyways the underlying database structures do the right thing here and take care of locking and everything. This allows us to merge the betamax fixture into the feed_manager one, which is necessary to get a (properly setup) session to do HTTP requests. This removes the new sessionCache() hack that was required to pass the right db_path to the session configuration, which was the main goal of this refactoring.