transport.
Sending the msgNewKeys packet and setting up the key material
now happen under a lock, preventing races with concurrent
writers.
R=kardianos, agl, jpsugar, hanwenn
CC=golang-dev
https://golang.org/cl/14476043
Adds readPacket() to conn, and renames conn to packetConn.
Key exchanges operate on packetConn, so they can be
unittested.
R=agl, jpsugar, dave
CC=golang-dev
https://golang.org/cl/13352055