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




Valid concern. However, since we use the SlotManager.class value when interning, the generated intern has much chance of being unique. (This will reach a deadlock only if there is another application synchronizing with the same class value.) 

However, there was a flaw in the synchronizing logic here. Synchronizing by the nodeID to modify the whole slotAssignmentMap collection is too heavy to be an atomic operation. Therefore I have modified this as follows : 

1. Retrieve all NodeIDs from HazelcastAgent
2. Iterate per nodeID and synchronize access to each entry in the slotAssignmentMap.