some refactoring (AsyncHttpClient#10)

* some refactoring

motivation: better code structure

changes:
* abstract errors as enum instead of nested structs
* make configurationa nested class

* errors as static let

* * move request, response, task to be nested under HTTPClient
* rename HTTPResponseDelegate -> HTTPClientResponseDelegate
* adjust tests

* Update SwiftNIOHTTP.swift

Author: tomerd

Sources/NIOHTTPClient/HTTPCookie.swift

@@ -110,7 +110,7 @@ public struct HTTPCookie {
     }
 }
 
-public extension HTTPResponse {
+public extension HTTPClient.Response {
     internal var cookieHeaders: [HTTPHeaders.Element] {
         return headers.filter { $0.name.lowercased() == "set-cookie" }
     }