Allow data adapters to return new refresh information after each refresh

Currently the data adapter API allows only for a fixed interval of refreshes, but some data sources actually expose more information on when a refresh might yield new information, notably HTTP cache headers.

If we could pass that information to the [`LookupDataAdapterRefreshService`](https://github.com/Graylog2/graylog2-server/blob/master/graylog2-server/src/main/java/org/graylog2/lookup/LookupDataAdapterRefreshService.java) then we could improve reloading to expose more up-to-date information without excessive polling.