The broker shuts itself down because it needs a persistency store. Without one it cannot guarantee message delivery for incoming traffic while the database is down. Currently, I don't think that the design allows for a broker to continue running without a storage.
If you would like the broker to automatically start again automatically you could wrap the broker service in a POJO where you use Spring to instantiate the broker service. In a loop you test the state of the broker, if it shuts down you poll the database until it's up again, at which point you can start the broker service again.