U4-4931 - 6.2 - lock contention & potential application lockdown

See details in issue tracker, this is #384 and #388 re-targeted for 6.2(.2).
Yeah, will probably make sense to get rid of the DictionaryCacheProviderBase (my bad, thought it'd be good to share code but in hindsight not really in this case). I think that there probably shouldn't be a lock for this provider at all.