Support cache refreshing in the background

[ryanlchandler]

It would be useful to have the cache refresh in the background to avoid paying the cost of a cache miss. Coming from the Java world I'm thinking of something like Google's caching library - https://github.com/google/guava/wiki/CachesExplained#refresh

[cabol]

Looks interesting. Not sure if something similar can be achieved using the caching decorators. I'll definitely take a look at it. Thanks!