Fix all linting errors

Disabled gocylco for now, need to use gometalinter to disable
conditionally. There are also a lot more linters we could use, but they
cause too many issues to start today.

internal/datachannel/message_channel_ack.go

@@ -4,6 +4,7 @@ import (
 	"github.com/pkg/errors"
 )
 
+// ChannelAck is used to ACK a DataChannel open
 type ChannelAck struct{}
 
 const (