Default value of "merge-policy" xsd attribute should not be a lambda

A different lambda name gets generated during each build.
So an xsd created and deployed to http://docs.jboss.org/infinispan/schemas/ becomes out of sync after next build. We can fix that by redeploying the schema but we cannot assume users will always re-fetch it.

Using static inner classes (non-anonymous) would be preferable in this case even though the lambda code looks nicer. This solution maintains backward compat with 9.1 too.