shrpx: Fix blocking upstream RST_STREAM and propagate REFUSED_STREAM

This change fixes upstream RST_STREAM is blocked until
SpdyUpstream::send() is called. Now downstream REFUSED_STREAM is
propagated to upstream client so that client can reset request.  The
RST_STREAM error code when downstream went wrong is changed from
CANCEL to INTERNAL_ERROR.
shrpx: Fix WINDOW_UPDATE may block until SpdyUpstream::send()

spdy_data_read_callback in SpdyDownstreamConnection calls
SpdyUpstream::resume_read() which submits WINDOW_UPDATE, but after
that they are not call SpdyUpstream::send(). This means that if no
pending outgoing data in upstream, then WINDOW_UPDATE is blocked until
SpdyUpstream::send() from somewhere. This change adds
SpdyUpstream::send() to resume_read() so that WINDOW_UPDATE is not
blocked.