fs/p9p/new: various client and server cleanup Signed-off-by: Stephen J Day <stephen.day@docker.com>

fs/p9p/new: cleanup encoding/decoding We take some time to cleanup the codec implementation. The need for the message to implement the Message interface has been removed to allow flexibility in what can land in the message field. Error types can be returned directly now. The messages have been moved into a separate file. We also ensure that the number of pointer types is reduced in deserialized messages, hopefully avoiding unnecessary pointer traversal. Signed-off-by: Stephen J Day <stephen.day@docker.com>