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




Synchronized blocks on String literals may lead to potential deadlocks [1][2]. 
Interning string makes it a global object and if you use the same interned string on different parts of the code, this might lead to potential deadlock.

[1] https://octaviansima.wordpress.com/tag/string-intern/
[2] http://www.javalobby.org/java/forums/t96352.html