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>

images/image.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"io/ioutil"
+	"time"
 
 	"github.com/containerd/containerd/content"
 	digest "github.com/opencontainers/go-digest"
@@ -13,14 +14,21 @@ import (
 
 // Image provides the model for how containerd views container images.
 type Image struct {
-	Name   string
-	Target ocispec.Descriptor
+	Name                 string
+	Labels               map[string]string
+	Target               ocispec.Descriptor
+	CreatedAt, UpdatedAt time.Time
 }
 
 type Store interface {
-	Update(ctx context.Context, name string, desc ocispec.Descriptor) error
 	Get(ctx context.Context, name string) (Image, error)
-	List(ctx context.Context) ([]Image, error)
+	List(ctx context.Context, filters ...string) ([]Image, error)
+	Create(ctx context.Context, image Image) (Image, error)
+
+	// Update will replace the data in the store with the provided image. If
+	// one or more fieldpaths are provided, only those fields will be updated.
+	Update(ctx context.Context, image Image, fieldpaths ...string) (Image, error)
+
 	Delete(ctx context.Context, name string) error
 }