QNetworkReply::setReadBufferSize fix for threaded http

QNetworkReply::setReadBufferSize fix for threaded http

Added the setReadBufferSize functionallity again by limiting the
amount that the delegate read from the channel. Each time that data is
fetched from the reply buffer, we communicate back to the thread so
that more data can be fetched.

Task-number: QTBUG-25327
Change-Id: I2f9950196e64acd09bc8da50c1116f2c9deacad4
Reviewed-by: Shane Kearns <shane.kearns@accenture.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>