MEDIUM: h2: process streams pending for sending

MEDIUM: h2: process streams pending for sending

The send() callback calls h2_process_mux() which iterates over the list
of flow controlled streams first, then streams waiting for room in the
send_list. If a stream from the send_list ends up being flow controlled,
it is then moved to the fctl_list. This way we can maintain the most
accurate fairness by ensuring that flows are always processed in order
of arrival except when they're blocked by flow control, in which case
only the other ones may pass in front of them.

It's a bit tricky as we want to remove a stream from the active lists
if it doesn't block (ie it has no reason for staying there).