Add cancellation support when listing directory entries...

[sean-]

See #69 (comment) for details. Without cancellation, it's possible for the scanner.Scan() to block indefinitely attempting to wait for more data. The loop will end when the TCP connection is closed, but this should obey normal timeout/deadline semantics.

https://github.com/joyent/triton-go/blob/master/storage/directory.go#L65-L74