[MM-32390] Config logic refactor (#17578)

* Replace config generator

* Cleanup

* Some renaming and docs additions to add clarity

* Cleanup logging related methods

* Cleanup emitter

* Fix TestDefaultsGenerator

* Move feature flags synchronization logic out of config package

* Remove unnecessary util functions

* Simplify load/set logic

* Refine semantics and add some test to cover them

* Remove unnecessary deep copies

* Improve logic further

* Fix license header

* Review file store tests

* Fix test

* Fix test

* Avoid additional write during initialization

* More consistent naming

* Update app/feature_flags.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

app/feature_flags.go

@@ -8,7 +8,7 @@ import (
 	"os"
 	"time"
 
-	"github.com/mattermost/mattermost-server/v5/config"
+	"github.com/mattermost/mattermost-server/v5/app/featureflag"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
 )
 
@@ -21,7 +21,7 @@ func (s *Server) setupFeatureFlags() {
 	splitConfigured := splitKey != ""
 	syncFeatureFlags := splitConfigured && s.IsLeader()
 
-	s.configStore.PersistFeatures(splitConfigured)
+	s.configStore.SetReadOnlyFF(!splitConfigured)
 
 	if syncFeatureFlags {
 		if err := s.startFeatureFlagUpdateJob(); err != nil {
@@ -71,7 +71,7 @@ func (s *Server) startFeatureFlagUpdateJob() error {
 		attributes["group_id"] = groupId
 	}
 
-	synchronizer, err := config.NewFeatureFlagSynchronizer(config.FeatureFlagSyncParams{
+	synchronizer, err := featureflag.NewSynchronizer(featureflag.SyncParams{
 		ServerID:   s.TelemetryId(),
 		SplitKey:   *s.Config().ServiceSettings.SplitKey,
 		Log:        log,