Introduce QUIC_VERSION_20: allowing endpoints to set different

Introduce QUIC_VERSION_20: allowing endpoints to set different
stream/session flow control windows. Two new tags kSFCW, kCFCW for
stream flow control window and session (connection) flow control windows
respectively. kIFCW is still used for older versions <= QUIC_VERSION_19.
kIFCW has the same value as kCFCW.

Most of this CL is copy/paste code for setting/getting the new tags. New
values are set in quic_client_bin.cc (and quic_stream_factory.cc), and
processed in quic_session.cc and reliable_quic_stream.cc

rtenneti: When merging into Chromium, you should add something like:

config.SetInitialStreamFlowControlWindowToSend(kInitialReceiveWindow);
config.SetInitialSessionFlowControlWindowToSend(kInitialReceiveWindow);

just before *session = new QuicClientSession(...) in
quic_stream_factory.cc

Introduce QUIC_VERSION_20: allowing endpoints to set different
stream/session flow control windows.

Merge internal change: 69557631

R=rch@chromium.org, rjshade@google.com

[Ryan Hamilton, 2014-06-19 23:55:15]

lgtm