The payload of a data message is defined as an SSH string type,
which uses the first four bytes to encode its length. When channelData
and channelExtendedData were added I defined Payload as []byte to
be able to use it directly without a string to []byte conversion. This
resulted in the length data leaking into the payload data.

This CL fixes the bug, and restores agl's original fast path code.

Additionally, a bug whereby s.lock was not released if a packet arrived
for an invalid channel has been fixed.

Finally, as they were no longer used, I have removed
the channelData and channelExtedendData structs.

R=agl, rsc
CC=golang-dev
https://golang.org/cl/5330053