Increase UDP buffer size

[pietdevaere]

The buffer size of the UDP sockets used by minq should be increased. On my machine (a recent Debian version) the buffers overflow when there is a high amount of ACK traffic. (a minimum amount of buffer space is allocated per packet, when there are a lot of small ACK packets, this causes overflows, even with proper congestion control). I am not quite sure if minq should do this internally (in udp_transport.go), or if it should be handled by the user (in bin/{server|client}/main.go)