MEDIUM: mux-h1: Revamp the way subscriptions are handled.

MEDIUM: mux-h1: Revamp the way subscriptions are handled.

Don't always wake the tasklets subscribed to recv or send events as soon as
we had any I/O event, and don't call the wake() method if there were no
subscription, instead, wake the recv tasklet if we received data in h2_recv(),
and wake the send tasklet if we were able to send data in h2_send(), and the
buffer is not full anymore.
Only call the data_cb->wake() method if we get an error/a read 0, just in
case the stream was not subscribed to receive events.