QUIC: do not call shutdown() when handshake is in progress.

QUIC: do not call shutdown() when handshake is in progress.

Instead, when worker is shutting down and handshake is not yet completed,
connection is terminated immediately.

Previously the callback could be called while QUIC handshake was in progress
and, what's more important, before the init() callback.  Now it's postponed
after init().

This change is a preparation to postponing HTTP/3 session creation to init().