Add cxy_cache:refresh_item/2,3

- Major rewrite to provide safety against unexpectedly disappearing ets tables for cache generations.
- Addition of refresh_item to avoid using delete_item for cache invalidation
- Added refresh counts and moved the ets records to a header file
- Refactored fetch, refresh to share access_item for consistent access to generation tables
- Added more extensive tests, including the first PropEr tests for validating atoms as cache names

This version should be tagged 0.9.8c