enhance: split config from server package

The github.com/containerd/containerd/services/server has a lot of
dependencies, like content, snapshots services implementation and
docker-metrics.

For the client side, it uses the config struct from server package
to start up the containerd in background. It will import a lot of
useless packages which might be conflict with existing vendor's package.

It makes integration easier with single config package.

Signed-off-by: Wei Fu <fuweid89@gmail.com>

services/server/server_test.go

@@ -20,13 +20,14 @@ import (
 	"context"
 	"testing"
 
+	srvconfig "github.com/containerd/containerd/services/server/config"
 	"gotest.tools/assert"
 	is "gotest.tools/assert/cmp"
 )
 
 func TestNewErrorsWithSamePathForRootAndState(t *testing.T) {
 	path := "/tmp/path/for/testing"
-	_, err := New(context.Background(), &Config{
+	_, err := New(context.Background(), &srvconfig.Config{
 		Root:  path,
 		State: path,
 	})