QUIC: further limiting maximum QUIC packet size.

QUIC: further limiting maximum QUIC packet size.

quic-transport draft 29, section 14:

    QUIC depends upon a minimum IP packet size of at least 1280 bytes.
    This is the IPv6 minimum size [RFC8200] and is also supported by most
    modern IPv4 networks.  Assuming the minimum IP header size, this
    results in a QUIC maximum packet size of 1232 bytes for IPv6 and 1252
    bytes for IPv4.

Since the packet size can change during connection lifetime, the
ngx_quic_max_udp_payload() function is introduced that currently
returns minimal allowed size, depending on address family.