optimize quic-go for short RTTs

[marten-seemann]

There are a number of things that would need to be done to make quic-go work well on short (<20ms) RTTs:

• To avoid retransmissions during the handshake, handshake packets need to be acknowledged immediately. This is just fixing a bug, RFC 9002 already requires us to do so: don't delay acknowledgement by max_ack_delay during the handshake #4279
• The max_ack_delay (25ms) is way too long for short RTTs, leading to sluggish loss recovery. We should be able to negotiate a shorter value. We'll need the ACK Frequency extension for that: implement the ack-frequency extensions #2482.
• Often times users know the expected RTT (e.g. when starting a transfer in a DC). It should be possible to configure the initial RTT to speed up the handshake: add a config option for the initial RTT #3754.