Implement remote instance entities caching strategy

[jum-s]

A caching strategy between servers provides a few advantages (non-exhaustive):

• reduce server2server number of requests,
• improve instance anonymization,
• avoids hitting inventaire.io's cache too much

Invalidation implementations:
The make-it-work version would trigger a refresh from instance server at periodical time (on the model of client cache refresh (TTL based)).
A make-it-right version may rely on pushing notification updates from inventaire.io, to keep up to date data across instances, while reducing requests between servers. This would required to store domains who already requested an entity(?)