storage integration

contains changes needed to integrate different storage modules to mb. (eg: cassandra, mysql, H2)

NOTE: merge after the code review is complete.
This is removed in commit https://github.com/wso2-dev/andes/commit/fd6529dec383d94e3522f8fe5e129ca15f109627/modules/andes-core/broker/src/main/java/org/wso2/andes/kernel/MessagingEngine.java