MEDIUM: quic: separate path for rx and tx with set_encryption_secrets

MEDIUM: quic: separate path for rx and tx with set_encryption_secrets

With quicTLS the set_encruption_secrets callback is always called with
the read_secret and the write_secret.

However this is not the case with libreSSL, which uses the
set_read_secret()/set_write_secret() mecanism. It still provides the
set_encryption_secrets() callback, which is called with a NULL
parameter for the write_secret during the read, and for the read_secret
during the write.

The exchange key was not designed in haproxy to be called separately for
read and write, so this patch allow calls with read or write key to
NULL.