QUIC: added support for segmentation offloading. To improve output performance, UDP segmentation offloading is used if available. If there is a significant amount of data in an output queue and path is verified, QUIC packets are not sent one-by-one, but instead are collected in a buffer, which is then passed to kernel in a single sendmsg call, using UDP GSO. Such method greatly decreases number of system calls and thus system load.