HTTP/2 Support

We need Qt 5.9 for HTTP2 because, even if Qt 5.8 already has support
for it, there is some critical bug in the HTTP2 implementation which
make it unusable [ https://codereview.qt-project.org/186050 and
https://codereview.qt-project.org/186066 ]

When using HTTP2, we can use many more parallel network request, this
is especially good for small file handling

Lower the priority of the GET and PUT propagation jobs, so the quota
or selective sync ui PROPFIND will not be blocked by them