Avoid use of SPDY stream ID 1, as that's typically for UPGRADE.

Typical use of HTTP/2 stream ID 1 is for [UPGRADE](http://tools.ietf.org/html/draft-ietf-httpbis-http2-10#section-5.1.1), which we don't support.  We found that Netty enforces stream ID 1 can't be used, so might as well switch.

This also moves SPDY/3 clients to start at stream ID 3 as well.