rework CacheWriter writeAll() and deleteAll() to permit partial success

I believe you're right about our code, at present, not being tripped up by a class issue in either `successes` or `failures` -- they might, at some point in time, be consumed by a ResilienceStrategy implementation but today they're not.  (Even then, I think the ResilienceStrategy would need to invoke some class-specific method or perform some explicit cast for an improperly classed key to cause an issue -- not likely for one we'd write.)

Ready to merge?