Optimize RAM usage with many databases open

[lvca]

When an ArcadeDB server has many databases installed (or even a JVM running ArcadeDB embedded), when the database is open, some operation during opening requires quite a lot of RAM. It would be more efficient if we apply some changes:

• [ ] Lazy load buckets/indexes, only at first access
• [ ] Avoid scanning the bucket to retrieve the holes, but do this operation at first insert/update/delete in the bucket

[robfrank]

@lvca is this still relevant?

[lvca]

Yes, we still need to do those things to improve startup time + initial RAM used