Use one callback for SYN_STREAM, SYN_REPLY and HEADERS.

This is closer to the semantics of HTTP/2.0, which uses the HEADERS
frame for all three types. SPDY is a bit more strict because it
relies on redundancy; it's an error to send a SYN_REPLY after a
HEADERS frame. With HTTP/2.0, there's only one type so there's
no error to detect.