content: cleanup service and interfaces

After implementing pull, a few changes are required to the content store
interface to make sure that the implementation works smoothly.
Specifically, we work to make sure the predeclaration path for digests
works the same between remote and local writers. Before, we were
hesitent to require the the size and digest up front, but it became
clear that having this provided significant benefit.

There are also several cleanups related to naming. We now call the
expected digest `Expected` consistently across the board and `Total` is
used to mark the expected size.

This whole effort comes together to provide a very smooth status
reporting workflow for image pull and push. This will be more obvious
when the bulk of pull code lands.

There are a few other changes to make `content.WriteBlob` more broadly
useful. In accordance with addition for predeclaring expected size when
getting a `Writer`, `WriteBlob` now supports this fully. It will also
resume downloads if provided an `io.Seeker` or `io.ReaderAt`. Coupled
with the `httpReadSeeker` from `docker/distribution`, we should only be
a lines of code away from resumable downloads.

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

cmd/dist/main.go

@@ -38,9 +38,11 @@ distribution tool
 			EnvVar: "CONTAINERD_FETCH_TIMEOUT",
 		},
 		cli.StringFlag{
+			// TODO(stevvooe): for now, we allow circumventing the GRPC. Once
+			// we have clear separation, this will likely go away.
 			Name:  "root",
 			Usage: "path to content store root",
-			Value: "/tmp/content", // TODO(stevvooe): for now, just use the PWD/.content
+			Value: "/var/lib/containerd",
 		},
 		cli.StringFlag{
 			Name:  "socket, s",