snapshot: fix race conditions

* perform collection.LoadComplete inside maybeRunTaskInBackground
 * have tasks use a fresh copy of taskCollectionFactory, taskCollection
 * fix locking for snapshots of snapshots by locking SourceSnapshots
 * use uuids, since names can be renamed

deb/snapshot.go

@@ -125,12 +125,6 @@ func (s *Snapshot) Key() []byte {
 	return []byte("S" + s.UUID)
 }
 
-// ResourceKey is a unique identifier of the resource
-// this snapshot uses. Instead of uuid it uses name
-// which needs to be unique as well.
-func (s *Snapshot) ResourceKey() []byte {
-	return []byte("S" + s.Name)
-}
 
 // RefKey is a unique id for package reference list
 func (s *Snapshot) RefKey() []byte {