Add WithVersion to context and other cleanup

By adding WithVersion to the context package, we can simplify context setup in
the application. This avoids some odd bugs where instantiation order can lead
to missing instance.id or version from log messages.

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

context/version_test.go

@@ -0,0 +1,19 @@
+package context
+
+import "testing"
+
+func TestVersionContext(t *testing.T) {
+	ctx := Background()
+
+	if GetVersion(ctx) != "" {
+		t.Fatalf("context should not yet have a version")
+	}
+
+	expected := "2.1-whatever"
+	ctx = WithVersion(ctx, expected)
+	version := GetVersion(ctx)
+
+	if version != expected {
+		t.Fatalf("version was not set: %q != %q", version, expected)
+	}
+}