QUIC: added stateless reset support.

QUIC: added stateless reset support.

The new "quic_stateless_reset_token_key" directive is added.  It sets the
endpoint key used to generate stateless reset tokens and enables feature.

If the endpoint receives short-header packet that can't be matched to
existing  connection, a stateless reset packet is generated with
a proper token.

If a valid stateless reset token is found in the incoming packet,
the connection is closed.

Example configuration:

http {
    quic_stateless_reset_token_key  "foo";
    ...
}