When a SPDY client, do not flow-control the server

When speaking to @jpinner, I found out that only servers or intermediaries have a use case for small flow control windows.  This pull request (effectively) disables flow control when a client, by setting the window update size to max (2GB).

With this change, a sample caliper bench of spdy/3.1 on my laptop recovered 13 requests per second with concurrency set to 10 and body size 1048576.