Utilitize HTTP caching for API resources

[k9delight]

We should be able to use HTTP Etags for API resources to reduce bandwidth.

Taking an etag from the client and comparing memcache.get(resource_type_md5) gives us the ability to quickly identify if the data has changed, and if to return the new results, alternatively send a 304 not modified if unchanged.

Benefits: bandwidth saved, maybe reduced CPU time?

Costs: more code paths added to the application

No clear consensus if this is worthwhile implementing vs adding other functionality.