pools.

 *  **OkHttpClient is now stateless.** In the 2.x API `OkHttpClient` had getters
    and setters. Internally each request was forced to make its own complete
    snapshot of the `OkHttpClient` instance to defend against racy configuration
    changes. In 3.x, `OkHttpClient` is now stateless and has a builder. Note
    that this class is not strictly immutable as it has stateful members like

 * }
 * ```
 *
 * This technique works even better in situations where a stale response is better than no response.
 * To permit stale cached responses, use the `max-stale` directive with the maximum staleness in
 * seconds:
 *
 * ```java
 * Request request = new Request.Builder()
 *     .cacheControl(new CacheControl.Builder()
 *         .maxStale(365, TimeUnit.DAYS)
 *         .build())