Add synchronous image delete

Synchronous image delete provides an option image delete to wait
until the next garbage collection deletes after an image is removed
before returning success to the caller.

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

cmd/containerd/builtins.go

@@ -3,7 +3,7 @@ package main
 // register containerd builtins here
 import (
 	_ "github.com/containerd/containerd/diff/walking"
-	_ "github.com/containerd/containerd/gc/policy"
+	_ "github.com/containerd/containerd/gc/scheduler"
 	_ "github.com/containerd/containerd/services/containers"
 	_ "github.com/containerd/containerd/services/content"
 	_ "github.com/containerd/containerd/services/diff"