Simplify URLRequestCustomJob handling

Improve implementation of URLRequestCustomJob:

* remove qmutex, pass values using PostTask
* do not use base::WeakPtr which is not thread safe and must
  always be dereferenced on the same thread
* add proxy object to handle interactions between threads
* do not use QPointer to track IODevice since it does not solve
  anything for us
* QIODevice in reply method is used only by IO thread
* do not make shared object to commit suicide,
  instead use refcounted object
* improve documentation about thread safety issue of QIODevice
  object in reply method

Change-Id: Ic29bf262de8082dfd46cb9217a68f3c982d16b9e
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>