images, containers: converge metadata API conventions

The primary feature we get with this PR is support for filters and
labels on the image metadata store. In the process of doing this, the
conventions for the API have been converged between containers and
images, providing a model for other services.

With images, `Put` (renamed to `Update` briefly) has been split into a
`Create` and `Update`, allowing one to control the behavior around these
operations. `Update` now includes support for masking fields at the
datastore-level across both the containers and image service. Filters
are now just string values to interpreted directly within the data
store. This should allow for some interesting future use cases in which
the datastore might use the syntax for more efficient query paths.

The containers service has been updated to follow these conventions as
closely as possible.

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

metadata/adaptors.go

@@ -0,0 +1,79 @@
+package metadata
+
+import (
+	"strings"
+
+	"github.com/containerd/containerd/containers"
+	"github.com/containerd/containerd/filters"
+	"github.com/containerd/containerd/images"
+)
+
+func adaptImage(o interface{}) filters.Adaptor {
+	obj := o.(images.Image)
+	return filters.AdapterFunc(func(fieldpath []string) (string, bool) {
+		if len(fieldpath) == 0 {
+			return "", false
+		}
+
+		switch fieldpath[0] {
+		case "name":
+			return obj.Name, len(obj.Name) > 0
+		case "target":
+			if len(fieldpath) < 2 {
+				return "", false
+			}
+
+			switch fieldpath[1] {
+			case "digest":
+				return obj.Target.Digest.String(), len(obj.Target.Digest) > 0
+			case "mediatype":
+				return obj.Target.MediaType, len(obj.Target.MediaType) > 0
+			}
+		case "labels":
+			return checkMap(fieldpath[1:], obj.Labels)
+			// TODO(stevvooe): Greater/Less than filters would be awesome for
+			// size. Let's do it!
+		}
+
+		return "", false
+	})
+}
+func adaptContainer(o interface{}) filters.Adaptor {
+	obj := o.(containers.Container)
+	return filters.AdapterFunc(func(fieldpath []string) (string, bool) {
+		if len(fieldpath) == 0 {
+			return "", false
+		}
+
+		switch fieldpath[0] {
+		case "id":
+			return obj.ID, len(obj.ID) > 0
+		case "runtime":
+			if len(fieldpath) <= 1 {
+				return "", false
+			}
+
+			switch fieldpath[1] {
+			case "name":
+				return obj.Runtime.Name, len(obj.Runtime.Name) > 0
+			default:
+				return "", false
+			}
+		case "image":
+			return obj.Image, len(obj.Image) > 0
+		case "labels":
+			return checkMap(fieldpath[1:], obj.Labels)
+		}
+
+		return "", false
+	})
+}
+
+func checkMap(fieldpath []string, m map[string]string) (string, bool) {
+	if len(m) == 0 {
+		return "", false
+	}
+
+	value, ok := m[strings.Join(fieldpath, ".")]
+	return value, ok
+}