redundancy with WINDOW_UPDATE and SETTINGS_FLOW_CONTROL_OPTIONS

There's a WINDOW_UPDATE flag (
http://http2.github.io/http2-spec/#EndFlowControl) for ending flow control
for the session or individual stream. There's also
a SETTINGS_FLOW_CONTROL_OPTIONS for ending flow control for the session and
for all streams.

I think all these knobs are useful, but the redundancy is unnecessary. How
about we make SETTINGS_FLOW_CONTROL_OPTIONS specific to stream flow control
and remove the session flag? To disable flow control for the session, use
WINDOW_UPDATE with the end flow control flag and stream id 0.