Introduce a cache for document translation locales

Getting the translation locales of a document currently always triggers a database query. As document locales for the same document are queried many times in the Solr indexer (for every object, object property, and attachment of a document) and also shown in the UI, it would be good to introduce a cache for them. The cache should be cleared when the list of enabled locales is changed and invalidated for a document when a document with that reference (regardless of the locale) is created or deleted.

This cache should reduce the time needed to get a document for Solr indexing by about 10%.