[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/email_batching.go

@@ -171,13 +171,11 @@ func (job *EmailBatchingJob) checkPendingNotifications(now time.Time, handler fu
 
 		// get how long we need to wait to send notifications to the user
 		var interval int64
-		pchan := job.server.Store.Preference().Get(userId, model.PREFERENCE_CATEGORY_NOTIFICATIONS, model.PREFERENCE_NAME_EMAIL_INTERVAL)
-		if result := <-pchan; result.Err != nil {
+		preference, err := job.server.Store.Preference().Get(userId, model.PREFERENCE_CATEGORY_NOTIFICATIONS, model.PREFERENCE_NAME_EMAIL_INTERVAL)
+		if err != nil {
 			// use the default batching interval if an error ocurrs while fetching user preferences
 			interval, _ = strconv.ParseInt(model.PREFERENCE_EMAIL_INTERVAL_BATCHING_SECONDS, 10, 64)
 		} else {
-			preference := result.Data.(model.Preference)
-
 			if value, err := strconv.ParseInt(preference.Value, 10, 64); err != nil {
 				// // use the default batching interval if an error ocurrs while deserializing user preferences
 				interval, _ = strconv.ParseInt(model.PREFERENCE_EMAIL_INTERVAL_BATCHING_SECONDS, 10, 64)