Memoizer refactoring

Reduces the amount of refactoring to fix http://trac.openmicroscopy.org.uk/ome/ticket/10750

Memoizer is instantiated in a minimal number of places, but the usage of OMEROWrapper and storage of various readers within the stack are effectively untouched.  We will still want to do that refactoring in the near future, but for now this should give us caching without display bugs (at least, it works for me).