common.go:
* simplify findAgreedAlgorithms.
* add channelExtendedData support.

messages.go:
* add clientExtendedData.

server.go:
*  use simplified findAgreedAlgorithms.

server_shell.go:
* fix shadowed err return value.

transport.go:
* introduce separate cipher, mac and compression for each direction.
* added filteredConn and packetWriter interfaces.
* newTransport requires a source of randomness.

R=golang-dev, agl, rsc
CC=golang-dev
https://golang.org/cl/5285044