[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/featureflag/split_logger.go

@@ -0,0 +1,35 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package featureflag
+
+import (
+	"fmt"
+
+	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+)
+
+type splitLogger struct {
+	wrappedLog *mlog.Logger
+}
+
+func (s *splitLogger) Error(msg ...interface{}) {
+	s.wrappedLog.Error(fmt.Sprint(msg...))
+}
+
+func (s *splitLogger) Warning(msg ...interface{}) {
+	s.wrappedLog.Warn(fmt.Sprint(msg...))
+}
+
+// Ignoring more verbose messages from split
+func (s *splitLogger) Info(msg ...interface{}) {
+	//s.wrappedLog.Info(fmt.Sprint(msg...))
+}
+
+func (s *splitLogger) Debug(msg ...interface{}) {
+	//s.wrappedLog.Debug(fmt.Sprint(msg...))
+}
+
+func (s *splitLogger) Verbose(msg ...interface{}) {
+	//s.wrappedLog.Info(fmt.Sprint(msg...))
+}