Investigate un-_workList protected access to _work object

During investigation of JBTM-2865 it was detected that the _work object can be accessed outside of the _workList synchronized block. Notably this seems to be a .remove() operation on it which can mutate the struct and may cause issues.

For example, this looks wrong:
https://github.com/tomjenkinson/narayana/blob/adda493b7bbd030eb405e3ef20978dc5d30ac5c2/ArjunaCore/arjuna/classes/com/arjuna/ats/internal/arjuna/objectstore/CacheStore.java#L703

If
https://github.com/tomjenkinson/narayana/blob/adda493b7bbd030eb405e3ef20978dc5d30ac5c2/ArjunaCore/arjuna/classes/com/arjuna/ats/internal/arjuna/objectstore/CacheStore.java#L187