[MM-15297] Migrate "Preference.Get" to Sync by default #10721 (#10811)

* modify Get method in preference_store

* modify interface to match changes on preference store Get method

* modify variable initialization

* fix behavior on GetPreferenceByCategoryAndNameForUser

app/preference.go

@@ -32,13 +32,12 @@ func (a *App) GetPreferenceByCategoryForUser(userId string, category string) (mo
 }
 
 func (a *App) GetPreferenceByCategoryAndNameForUser(userId string, category string, preferenceName string) (*model.Preference, *model.AppError) {
-	result := <-a.Srv.Store.Preference().Get(userId, category, preferenceName)
-	if result.Err != nil {
-		result.Err.StatusCode = http.StatusBadRequest
-		return nil, result.Err
+	res, err := a.Srv.Store.Preference().Get(userId, category, preferenceName)
+	if err != nil {
+		err.StatusCode = http.StatusBadRequest
+		return nil, err
 	}
-	data := result.Data.(model.Preference)
-	return &data, nil
+	return res, nil
 }
 
 func (a *App) UpdatePreferences(userId string, preferences model.Preferences) *model.AppError {