Add java.time.Duration-based APIs

Representing date/time concepts as numeric primitives is a bad idea. It leads to unit mismatch, overflows, and many other dangerous date/time-related bugs.

For instance, it's impossible to tell what time unit `readTimeout` is without reading it's javadocs:
```java
public final class HttpRequest {
public HttpRequest setReadTimeout(int readTimeout) { ... }
}
```

Please consider adding `java.time.Duration` overloads to all APIs that accept a date/time concept as a numeric primitive. Methods that _return_ a numeric primitive representing a date/time concept are much more difficult as new, alternative method names must be chosen (e.g., `public int getReadTimeout()`).