fix contrib/ltr custom inner class feature/normaliser/model persistence

Custom feature/normaliser/model persistence across reloads is broken if the custom class is an inner class. This is because the class would be configured as (say) com.company.solr.ltr.feature.CustomFeatures$FeatureFoo but the stores (wrongly) persist com.company.solr.ltr.feature.CustomFeatures.FeatureFoo i.e. it's about the difference between getName() and getCanonicalName() – helpful link: https://stackoverflow.com/questions/15202997/what-is-the-difference-between-canonical-name-simple-name-and-class-name-in-jav/15203417