Fix compatibility with Rails edge

[casperisfine]

Following rails/rails#42649 we need to make some small changes to fix with_local_store.

Now ActiveSupport use the local store as a "payload" store (e.g. it contain serialized payloads instead of Entry instance). Both upstream MemCacheStore and MemcachedStore don't play so well with this new architecture, because they directly pass Entry instance to the underlying client.

So we basically need to backport the old local store behavior that was duping the cache entry to avoid leaks.

Longer term we should migrate to the new architecture where we no longer rely on memcached serializing objects for us (See: #63).

[shioyama]

These changes are not applied when Rails assigns LocalStore in middleware here:

https://github.com/rails/rails/blob/c903dfe618b8ed2b2c6d73b162a49fc4d478a855/activesupport/lib/active_support/cache/strategy/local_cache_middleware.rb#L28

            LocalCacheRegistry.set_cache_for(local_cache_key, LocalStore.new)

This means that duping is broken for middleware-applied LocalCache when memcached_store is used, since the duping fixes above are not present in ActiveSupport::Cache::Strategy::LocalCache::LocalStore.

[byroot]

Yup. I'll need to use a decorator or something like that instead.