Allow for anchor net in regression testing.

[AlexisOlson]

To allow anchor net to be avaiable to test against similar to the code in lczero-server/main.go here:

[...]
	// Regression tests for current best.  Done here because the 'end of match' code logic isn't thread safe, so could create matches multiple times.
	var prevNetwork1 db.Network
	err = db.GetDB().Where("network_number = ?", bestNetwork.NetworkNumber-3).First(&prevNetwork1).Error
	if err == nil {
		createMatch(trainingRun, &prevNetwork1, true, string(params[:]))
	}
	var prevNetwork2 db.Network
	err = db.GetDB().Where("network_number = ?", bestNetwork.NetworkNumber-10).First(&prevNetwork2).Error
	if err == nil {
		createMatch(trainingRun, &prevNetwork2, true, string(params[:]))
	}
[...]

the desired network would need to be available. The following code in lczero-client/lc0_main.go deletes old networks:

[...]
func removeAllExcept(dir string, sha string, keepTime string) error {
	files, err := ioutil.ReadDir(dir)
	if err != nil {
		return err
	}
	for _, file := range files {
		if file.Name() == sha {
			continue
		}
		timeLimit, _ := time.ParseDuration(keepTime)
		if time.Since(file.ModTime()) < timeLimit {
			continue
		}
		fmt.Printf("Removing %v\n", file.Name())
		err := os.RemoveAll(filepath.Join(dir, file.Name()))
		if err != nil {
			return err
		}
	}
	return nil
}
[...]

However, it should be relatively simple to include a condition to not delete specific networks. E.g.

		if file.Name() == "5c222ccd1ccbed2666b3a8ef94d8833d386d2168d51c3e99a0a3fc37a56d2569" {
			continue
		}

It would probably be better to store the sha in a variable declaration or a configuration file.

[borg323]

Since we already have the keepTime value from the server, the easiest thing to do is use os.Chtimes() to update the modification time of the net when used for a match. So every net that is used inside the keepTime window will not be deleted, without any hard-coding.
An alternative is to periodically get a touch value from the server and update the modification time of the specified net (if present).

[AlexisOlson]

So would it make sense to use Chtimes in the if testonly bit of the createMatch function or is there a better place to do that?

func createMatch(trainingRun *db.TrainingRun, network *db.Network, testonly bool, params string) error {
	match := db.Match{
		TrainingRunID: trainingRun.ID,
		CandidateID:   network.ID,
		CurrentBestID: trainingRun.BestNetworkID,
		Done:          false,
		GameCap:       config.Config.Matches.Games,
		Parameters:    params,
	}
	if testonly {
		match.TestOnly = true
	}
	return db.GetDB().Create(&match).Error
}

[borg323]

I think the best place is inside getNetwork(), right before https://github.com/LeelaChessZero/lczero-client/blob/master/lc0_main.go#L725