Provide upgrade path for deployments

There does not appear to be a documented upgrade process for Marmotta.

Using a non-volatile config, with a postgres backend - if you undeploy the war and copy a new one it it seems to work - after restarting Tomcat.

If you access before restarting you get errors:

java.lang.RuntimeException: java.sql.SQLException: persistence backend not initialized; call initialise before acquiring a connection
org.apache.marmotta.kiwi.sail.KiWiValueFactory.aqcuireConnection(KiWiValueFactory.java:90)