Enable HTTP client cache directory

Various subsytems load data asynchronously via HTTP.
These requests are currently not cached by the clients in Graylog, leading to some of the users implement half-baked conditional requests and caches, complicating the implementations.

okhttp already supports a [comprehensive on-disk LRU cache](https://github.com/square/okhttp/blob/master/samples/guide/src/main/java/okhttp3/recipes/CacheResponse.java) that is bounded in size.
Add configuration options (preferably changable on the fly), metrics and management capabilities to Graylog to improve the HTTP client subsystem.