Obsolete `IProxyFactoryFactory.IsProxy` and `NHibernateProxyHelper.IsProxy` methods and replace their usages

[hazzik]

[danfma]

Would be possible to add custom interface markers from the domain to the proxy? I think this would be nice to hide the NH from the domain layer.

interface ILazy { }

// then when using that somewhere
if (root.Property is ILazy) {
  // do something
}