MEDIUM: h2: enable reading again on the connection if it was blocked on stream buffer...

MEDIUM: h2: enable reading again on the connection if it was blocked on stream buffer full

If the polling update function is called with RD_ENA while H2_CF_DEM_SFULL
indicates the demux had to block on a stream buffer full condition, we can
remove the flag and re-enable polling for receiving because this is the
indication that a consumer stream has made some room in the buffer. Probably
that we should improve this to ensure that h2s->id == h2c->dsi and avoid
trying to receive multiple times in a row for the wrong stream.