BUG/MEDIUM: quic: Let it be known if the tasklet has been released.

BUG/MEDIUM: quic: Let it be known if the tasklet has been released.

quic_conn_release() may, or may not, free the tasklet associated with
the connection. So make it return 1 if it was, and 0 otherwise, so that
if it was called from the tasklet handler itself, the said handler can
act accordingly and return NULL if the tasklet was destroyed.
This should be backported if 9240cd4a2771245fae4d0d69ef025104b14bfc23
is backported.