MINOR: quic: Modify NEW_TOKEN frame structure (qf_new_token struct)

MINOR: quic: Modify NEW_TOKEN frame structure (qf_new_token struct)

Modify qf_new_token structure to use a static buffer with QUIC_TOKEN_LEN
as size as defined by the token for future connections (quic_token.c).
Modify consequently the NEW_TOKEN frame parser (see quic_parse_new_token_frame()).
Also add comments to denote that the NEW_TOKEN parser function is used only by
clients and that its builder is used only by servers.