Add indirection for large version vectors. (#6376)

This adds indirection of large version vectors in the same manner as we
already to block lists. The effect is the same: less duplicated data in
some situations.

To mitigate the impact for when this indirection
wouldn't be needed I've added an indirection cutoff for both blocks and
the new version vector stuff: we don't do the indirection at all for
small block lists or small version vectors, instead storing it directly
like we used to do. This is faster for small files and small setups.

lib/model/model.go

@@ -924,6 +924,7 @@ func (m *model) handleIndex(deviceID protocol.DeviceID, folder string, fs []prot
 		// The local attributes should never be transmitted over the wire.
 		// Make sure they look like they weren't.
 		fs[i].LocalFlags = 0
+		fs[i].VersionHash = nil
 	}
 	files.Update(deviceID, fs)
 
@@ -1968,7 +1969,10 @@ func (s *indexSender) sendIndexTo(ctx context.Context) error {
 		if f.IsReceiveOnlyChanged() {
 			f.Version = protocol.Vector{}
 		}
-		f.LocalFlags = 0 // never sent externally
+
+		// never sent externally
+		f.LocalFlags = 0
+		f.VersionHash = nil
 
 		previousWasDelete = f.IsDeleted()