Add special handling for cache TTL < 1 second

Add a decorator to CacheAccessor that makes sure we don't accidentally cache anything with a TTL of zero, i.e. infinity in Memcached-land.
I agree with @mauhiz I think we should just put it into `MemcachedAccessor`, since that layer is/should be acting as an "adapter" layer for the underlying `MemcachedClient`.