4.0.0 RC6

Note: This is a pre-release, there might be breaking changes in the final stable version.

• Connections are now attempted concurrently except for the first connection to a specific host if TLS is used, as that connection might be able to use HTTP/2
• Improved Response::setBody() API
• Improved backpressure mechanism for HTTP/2
• Changed Client and DefaultConnectionPool to be immutable, adding an interceptor returns a new instance now
• Client follows redirects by default, with an option to disable
• Client automatically retries unprocessed requests on properly closed HTTP/2 connections and idempotent requests on all connections up to two times now, the limit is customizable
• Added support for request attributes, a request local mutable storage for custom application / interceptor data
• Removed unused DnsException