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>

metadata/db_test.go

@@ -235,7 +235,7 @@ func TestMetadataCollector(t *testing.T) {
 		t.Fatalf("Creation failed: %+v", err)
 	}
 
-	if err := mdb.GarbageCollect(ctx); err != nil {
+	if _, err := mdb.GarbageCollect(ctx); err != nil {
 		t.Fatal(err)
 	}
 
@@ -322,7 +322,7 @@ func benchmarkTrigger(n int) func(b *testing.B) {
 
 				//b.StartTimer()
 
-				if err := mdb.GarbageCollect(ctx); err != nil {
+				if _, err := mdb.GarbageCollect(ctx); err != nil {
 					b.Fatal(err)
 				}