nghttpx: Fix connection preface is not read by upstream_http2_connhd_readcb

It seems that if readcb is not set before SSL/TLS handshake, the
incoming data already available when eventcb (BEV_EVENT_CONNECTED
event) is fired is not further notified after setting new readcb.  We
knew this fact and call upstream->on_read() in eventcb, but it is
wrong for HTTP/2.  We have to call upstream_http2_connhd_readcb to
check connection preface.  Otherwise, we consume it by nghttp2 session
and it is treated as unknown frame and connection preface is not
detected properly.