Housekeeping

As the year-end is approaching, let's join efforts and do some housekeeping :)

This PL is a collection of various fixes / cleanups (see individual commits) that did not yet found their way back upstream
please read the corresponding commit message: "make suffler class part of session key to prevent crash when switching shufflers that have different cache data format"

we have tested some different shuffler implementation that do not return an array (but a hash) when saving to the session (```save_session```). When we then switched the shufflers, the session data did restore the wrong store (array instead of hash or vice versa)