Add gc policy plugin

Add garbage collection as a background process and policy
configuration for configuring when to run garbage collection.
By default garbage collection will run when deletion occurs
and no more than 20ms out of every second.

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

plugin/plugin.go

@@ -54,6 +54,8 @@ const (
 	MetadataPlugin Type = "io.containerd.metadata.v1"
 	// ContentPlugin implements a content store
 	ContentPlugin Type = "io.containerd.content.v1"
+	// GCPlugin implements garbage collection policy
+	GCPlugin Type = "io.containerd.gc.v1"
 )
 
 // Registration contains information for registering a plugin