Use oslo-versionedobjects to provide schema validation for objects

At the current stage it's not a big deal for Searchlight to implement versionedobjects for rolling upgrades (thats why versionedobjects have been developed), but it can benefit from it, as versionedobjects provide system for schema validation of objects. It especially usefull for stores which are schemaless (MongoDB) and for this ones which have dynamic schema (Elasticsearch).