@Override
    protected String digestToStorageKey(final String key) {
        return keyHashingScheme.hash(key);
    }
Create a storage backend using the given <i>memcached</i> client and
applying the given cache configuration, serialization, and hashing
mechanisms.
@param client how to talk to <i>memcached</i>
@param config apply HTTP cache-related options
@param serializer alternative serialization mechanism
@param keyHashingScheme how to map higher-level logical "storage keys"
  onto "cache keys" suitable for use with memcached
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/memcached/MemcachedHttpAsyncCacheStorage.java