BUG/MINOR: quic: ignore STREAM after MUX closure on BE side

BUG/MINOR: quic: ignore STREAM after MUX closure on BE side

For frontend connections, quic_conn layer is able to reject any new
streams opened after MUX closure. This is necessary as the peer may not
have been notified yet of the closure.

This operation is unnecessary on backend side. This is due to the fact
that only HTTP protocols are currently supported on top of QUIC, with
requests initiated by the client. For requests started before the MUX
closure, either they are already completed or closed early with a
STOP_SENDING emitted during stream shut.

Prior to this patch, spurrious RESET_STREAM could have been emitted on
backend connections after MUX closure as quic_conn stream_max_bidi was
not correctly set. Now reject is only performed for frontend connections
so this should not occured anymore.

This should be backported up to 3.3.