Use more declarative prefix for the enhanced xml namespace in auto export

When Automatic Export creates delta XMLs, the enhanced XML namespace is declared with an h prefix, like:

xmlns:h="http://www.onehippo.org/jcr/xmlimport"

A more declarative prefix would be "esv" for Enhanced System View.

xmlns:esv="http://www.onehippo.org/jcr/xmlimport"