Playtesting Space is hanging frequently

(Bug, Investigate , Priority: Critical, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Summary: I am getting semi-frequent reports of "Querki is down" from SI Playtesters. That isn't usually quite true -- so far, in all cases the rest of the system has been okay -- but the Playtesting Space itself is clearly flaky as hell.
For now, I am focusing on gathering data, in the hopes of trying to puzzle out what is going on. So initially, the focus here is on the notes in the conversations below.
See Introduce a concept of multi-History for an extremely speculative idea about how we might fix this properly, if upgrading akka-persistence-cassandra doesn't do the trick.