cmd/dist, cmd/ctr: move image store access to GRPC

With this changeset, image store access is now moved to completely
accessible over GRPC. No clients manipulate the image store database
directly and the GRPC client is fully featured. The metadata database is
now managed by the daemon and access coordinated via services.

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

plugin/plugin.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"sync"
 
+	"github.com/boltdb/bolt"
 	"github.com/containerd/containerd"
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/snapshot"
@@ -32,7 +33,8 @@ type InitContext struct {
 	Root        string
 	State       string
 	Runtimes    map[string]containerd.Runtime
-	Store       *content.Store
+	Content     *content.Store
+	Meta        *bolt.DB
 	Snapshotter snapshot.Snapshotter
 	Config      interface{}
 	Context     context.Context