p9p: use new context package throughout

Signed-off-by: Stephen J Day <stephen.day@docker.com>

lint/vet: address several issues identified by lint/vet

Signed-off-by: Stephen J Day <stephen.day@docker.com>

errors: return errors rather than nil

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Modified defaultRWTimeout to 30

Signed-off-by: French Ben <frenchben@docker.com>

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Printing all requests and responses to the log is quite expensive, especially when sending megabytes of data through 9P. This patch leaves the logging in the error paths and the flush path. Signed-off-by: David Scott <dave.scott@docker.com>

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>

Signed-off-by: Stephen J Day <stephen.day@docker.com>

Signed-off-by: Stephen J Day <stephen.day@docker.com>