Investigate high retransmission rate of Snowflake

The recent report on Snowflake distinguishability caught the fact that Snowflake has a very high rate of retransmissions during the handshake: https://arxiv.org/pdf/2008.03254.pdf

This was reported to cause latency and it sounds like a bug. It could also apply to the rest of the connection after the handshake. We should look into this.