Intermediary refactor teasing apart http / ipc

Introduces an IPC struct and moves the logic out of the http handlers
and into methods on that.

common/messages/ipc.go

@@ -0,0 +1,18 @@
+package messages
+
+import (
+	"errors"
+)
+
+type Arg struct {
+	Body       []byte
+	RemoteAddr string
+	NatType    string
+}
+
+var (
+	ErrBadRequest  = errors.New("bad request")
+	ErrInternal    = errors.New("internal error")
+	ErrUnavailable = errors.New("service unavailable")
+	ErrTimeout     = errors.New("timeout")
+)