Improve i18n performance

[etj]

Before #13800, the MetadataManager loaded all the labels at once into the schema context, and retrieved the labels from the cache.
The label cache was checked for freshness at each schema context load, while now the check is performed as each request for label.
Since the check is implemented using the thesaurus date in the DB, reducing the checks means less DB accesses.

Improvement may be implemented by performing the check only once in a while.