Fixes some remaining issues now that QUIC recommends sending an ACK every two packets, mostly involving reverting to the TCP style TLP calculation.

I wrote this based on 'two full-sized packets' equalling a fixed 2400 bytes.  QUIC could send immediate acks for non-full-sized packets, but that's not what TCP does.  I could base it on the connection's packet size, but that requires the receiver to monitor the sender's max packet size, which made me a bit concerned?

Advice on how to ensure sender and receiver agree on packet size would be appreciated.