NewFailOverCache instantiates a fail-over cache NOTE: - exp1: level 1 cache expiration, e.g.

FailOverCache implements fail-over caching strategy via two-level cache.

RefreshFunc cache refresh function to retrieve the newest value, accepts a key as input which is also the cache key NOTE: - Handle function timeout carefully (better finish in no more than 2 seconds) - Avoid returning a nil value while error is nil too.