Do no allow arbitrarily large buffer during loading

If the network is faster or we get a cache-hit, we can currently
end up processing very large QByteArrays. To lower the maximum memory
pressure and improve caching, we should limit the maximum size of the
buffers.

Task-number: QTBUG-36979
Change-Id: I681e764d95db75aa846de2df8b84ac6630872afb
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>