BUG/MEDIUM: h2: Try to be fair when sending data.

BUG/MEDIUM: h2: Try to be fair when sending data.

On the send path, try to be fair, and make sure the first to attempt to
send data will actually be the first to send data when it's possible (ie
when the mux' buffer is not full anymore).
To do so, use a separate list element for the sending_list, and only remove
the h2s from the send_list/fctl_list if we successfully sent data. If we did
not, we'll keep our place in the list, and will be able to try again next time.

This should be backported to 1.9.