Improve multi-tenancy in `RepositoryCache`

[alexander-yevsyukov]

Now RepositoryCache (which belongs to the server.entity package) implements the caching basing on IdInTenant class (which belongs to server.tenant package). IdInTenant is a pair of ID of an entity and a TenantId to which the ID belongs.

Issues with the current implementations:

• We duplicate TenantId for all the IDs per tenant in the cache. It would be better to have cache slices.
• The cache treats multi-tenancy differently than storages. This is a bit confusing

Consider implementing generic cache with multi-tenancy support which can cache any type of things. Then, RepositoryCache can extend this generic cache, specifying generic parameter of the thing to store in cache to be Entity<I, ?>. This way we would not link the tenant package with the entity package.