MEDIUM: h2: enable connection polling for send when a cs wants to emit

MEDIUM: h2: enable connection polling for send when a cs wants to emit

A conn_stream indicates its intent to send by setting the WR_ENA flag
and calling mux->update_poll(). There's no synchronous write so the only
way to emit a response from a stream is to proceed this way. The sender
h2s is then queued into the h2c's send_list if it was not yet queued.

Once the connection is ready, it will enter its send() callback to visit
writers, calling their data->send_cb() callback to complete the operation
using mux->snd_buf().

Also we enable polling if the mux contains data and wasn't enabled. This
may happen just after a response has been transmitted using chk_snd().
It likely is incomplete for now and should probably be refined.