Add snapshot and diff service

Remove rootfs service in place of snapshot service. Adds
diff service for extracting and creating diffs. Diff
creation is not yet implemented. This service allows
pulling or creating images without needing root access to
mount. Additionally in the future this will allow containerd
to ensure extractions happen safely in a chroot if needed.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

api/services/snapshot/snapshots.proto

@@ -0,0 +1,81 @@
+syntax = "proto3";
+
+package containerd.v1.snapshot;
+
+import "gogoproto/gogo.proto";
+import "google/protobuf/empty.proto";
+import "github.com/containerd/containerd/api/types/mount/mount.proto";
+
+// Snapshot service manages snapshots
+service Snapshot {
+	rpc Prepare(PrepareRequest) returns (MountsResponse);
+	rpc View(PrepareRequest) returns (MountsResponse);
+	rpc Mounts(MountsRequest) returns (MountsResponse);
+	rpc Commit(CommitRequest) returns (google.protobuf.Empty);
+	rpc Remove(RemoveRequest) returns (google.protobuf.Empty);
+	rpc Stat(StatRequest) returns (StatResponse);
+	rpc List(ListRequest) returns (stream ListResponse);
+	rpc Usage(UsageRequest) returns (UsageResponse);
+	// "Snapshot" prepares a new set of mounts from existing name
+}
+
+message PrepareRequest {
+	string key = 1;
+	string parent = 2;
+}
+
+message MountsRequest {
+	string key = 1;
+}
+
+message MountsResponse {
+	repeated containerd.v1.types.Mount mounts = 1;
+}
+
+message RemoveRequest {
+	string key = 1;
+}
+
+message CommitRequest {
+	string name = 1;
+	string key = 2;
+}
+
+message StatRequest {
+	string key = 1;
+}
+
+enum Kind {
+	option (gogoproto.goproto_enum_prefix) = false;
+	option (gogoproto.enum_customname) = "Kind";
+
+	ACTIVE = 0 [(gogoproto.enumvalue_customname) = "KindActive"];
+
+	COMMITTED = 1 [(gogoproto.enumvalue_customname) = "KindCommitted"];
+}
+
+message Info {
+	string name = 1;
+	string parent = 2;
+	Kind kind = 3;
+	bool readonly = 4;
+}
+
+message StatResponse {
+	Info info = 1 [(gogoproto.nullable) = false];
+}
+
+message ListRequest{}
+
+message ListResponse {
+	repeated Info info = 1 [(gogoproto.nullable) = false];
+}
+
+message UsageRequest {
+	string key = 1;
+}
+
+message UsageResponse {
+	int64 inodes = 2;
+	int64 size = 1;
+}