Make QHttpServerRequest copyable

The QHttpServerRequest object is not copyable because it serves as both the parser and the request class. However, using QConcurrent::run() for route handlers that return QFuture<QHttpServerResponse> introduces a race condition. The afterRequestHandlers must retain the previous request state, but since only one request object is instantiated (in QHttpServerHttp[12]ProtocolHandler classes), it may be overwritten by other requests before afterRequestHandlersare executed.

Making QHttpServerRequest copyable would allow passing copies of request objects instead of duplicating individual components during QConcurrent::run() invocations. To achieve this, extract the parser state into a private QHttpServerParser class and make QHttpServerRequest copyable.