Clean up logging

Ensure that the server only logs in exceptional cases and prefixes each log line with the package name. Signed-off-by: Jonathan Rudenberg <jonathan@titanous.com>

channel: truncate twrite messages based on msize

While there are a few problems around handling of msize, the easiest to address and, arguably, the most problematic is that of Twrite. We now truncate Twrite.Data to the correct length if it will overflow the msize limit negotiated on the session. ErrShortWrite is returned by the `Session.Write` method if written data is truncated. In addition, we now reject incoming messages from `ReadFcall` that overflow the msize. Such messages are probably terminal in practice, but can be detected with the `Overflow` function. Tread is also handled accordingly, such that the Count field will be rewritten such that the response doesn't overflow the msize. Signed-off-by: Stephen J Day <stephen.day@docker.com>

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>