Broker.xml updated

- Reorganised MB store configuration properties
- Added Inbound events related configurations
hmm... the current comment states 
>This is the Task interval (seconds) for the content removal task which will remove  the actual message content from the store in the background. If the message rate is very high users can set this to a lower value

Comment is improved a bit with PR https://github.com/wso2-dev/carbon-business-messaging/pull/79