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/common.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"net"
+	"path/filepath"
+	"time"
+
+	"github.com/docker/containerd/content"
+	"github.com/urfave/cli"
+	"google.golang.org/grpc"
+)
+
+func resolveContentStore(context *cli.Context) (*content.Store, error) {
+	root := context.GlobalString("root")
+	if !filepath.IsAbs(root) {
+		var err error
+		root, err = filepath.Abs(root)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return content.NewStore(root)
+}
+
+func connectGRPC(context *cli.Context) (*grpc.ClientConn, error) {
+	socket := context.GlobalString("socket")
+	return grpc.Dial(socket,
+		grpc.WithBlock(),
+		grpc.WithInsecure(),
+		grpc.WithDialer(func(addr string, timeout time.Duration) (net.Conn, error) {
+			return net.DialTimeout("unix", socket, timeout)
+		}),
+	)
+}