Queue purge further refactoring - adding context to logs, missing comments

Refactoring on previous pull request : https://github.com/wso2-dev/andes/pull/144/

1. Since we have re-named the queues in stores to "storage queues", purge methods should align with that convention. the actual destination ("queueName") is inferred from "storageQueueName" where necessary. 

2. Added context information to logs. e.g. storageQueueName, purged timestamp 

3. Since we can retrieve data from JDBC stores in a stream-like manner (lazy loading of iterator), we do not need to send multiple paginated queries to fetch all message IDs in a queue. This was therefore changed in JDBCMessageStoreImpl

4. Added comments for method parameters.

5. Removed unused methods : 
MessageFlusher (a.k.a : QueueDeliveryWorker) : clearMessagesAccumilatedDueToInactiveSubscriptionsForQueue
MessagingEngine : moveMetaDataToQueue




Since here we are synchronizing the String along with the SlotManager class, fault mentioned in [2] will not happen as Hasitha has mentioned. On the other hand synchronizing the whole method will block other nodes calling the method. Since there are no restriction on accessing slotAssignmentMap cocurrently by different nodes, synchronized block is used on String along with the class name.