content: refactor content store for API

After iterating on the GRPC API, the changes required for the actual
implementation are now included in the content store. The begin change
is the move to a single, atomic `Ingester.Writer` method for locking
content ingestion on a key. From this, comes several new interface
definitions.

The main benefit here is the clarification between `Status` and `Info`
that came out of the GPRC API. `Status` tells the status of a write,
whereas `Info` is for querying metadata about various blobs.

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

cmd/dist/get.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"io"
+	"os"
+
+	digest "github.com/opencontainers/go-digest"
+	"github.com/urfave/cli"
+)
+
+var getCommand = cli.Command{
+	Name:      "get",
+	Usage:     "get the data for an object",
+	ArgsUsage: "[flags] [<digest>, ...]",
+	Description: `Display the paths to one or more blobs.
+	
+Output paths can be used to directly access blobs on disk.`,
+	Flags: []cli.Flag{},
+	Action: func(context *cli.Context) error {
+		cs, err := resolveContentStore(context)
+		if err != nil {
+			return err
+		}
+
+		dgst, err := digest.Parse(context.Args().First())
+		if err != nil {
+			return err
+		}
+
+		rc, err := cs.Open(dgst)
+		if err != nil {
+			return err
+		}
+		defer rc.Close()
+
+		_, err = io.Copy(os.Stdout, rc)
+		return err
+	},
+}