fast socket start

allow new sockets to be initialized with state from existing or recently existing connections to the same IP. This should give some performance improvements in the case of opening many sockets in sequence to the same IP, transferring in bulk.

what it does is to initialize the new socket with the same cwnd, mtu and ssthres as the existing (or recently existing) socket to the same IP. This way we may save the ramp-up of cwnd for the new socket.