MINOR: quic: notify connection layer on handshake completion

MINOR: quic: notify connection layer on handshake completion

Wake up connection layer on QUIC handshake completion via
quic_conn_io_cb. Select SUB_RETRY_RECV as this was previously unused by
QUIC MUX layer.

For the moment, QUIC MUX never subscribes for handshake completion.
However, this will be necessary for features such as the delaying of
early data forwarding via wait-for-handshake.

This patch will be necessary to implement wait-for-handshake support for
QUIC. As such, it must be backported with next commits up to 2.6,
after a mandatory period of observation.