• [API] Drop support for Q039.
• Improve ACK-queuing logic. Send an ACK once in a while if peer keeps on sending non-ack-eliciting packets.
• Improve Alt-Svc string: Q050 and later are not included in the old-style "quic" string.
• Send stateless resets if connection could not be promoted.
• Schedule MAX_DATA if needed when DATA_BLOCKED is received.
• Use ls-qpack 0.11.2 -- needed for server push optimization.
• Code cleanup: handle some error cases, improve logging.