[F93] (cache) reorder save_module to check lru_cache first

[peterjah]

• document all added functions
• try in sandbox /simulation/labnet
  • if part of node-launch, checked using the resync_check flag
• unit tests on the added/changed features
  • make tests compile
  • make tests pass
• add logs allowing easy debugging in case the changes caused problems
• if the API has changed, update the API specification

[Leo-Besancon]

LGTM, IMO the added test read counter on the hd cache could be removed to avoid bloating, but it's not too many lines so we can also keep it.