use normalized BinaryFen as evalCache id (lichess-org/lila#15020)

[niklasf]

• Use BinaryFen for eval cache id (use scalachess BinaryFen in evalCache lila#15020).
• Switch to new MongoDB collection: eval_cache2.
• Retire SmallFen entirely.

[ornicar]

Looking great, but I saw something sub-optimal in mongodb when trying it out:

db.eval_cache2.find()
[
  {
    _id: {
      position: Binary.createFromBase64('//8AAAAA//8thErSAAAAABERERE+lVvj', 0)
    },
    // [...]
  }
]

I think it should be _id: Binary.createFromBase64(...). Using an object as the primary key can't be efficient :)

[ornicar]

I can take care of it in a few hours.

[niklasf]

Oops. Glanced right over that. Fixed.

[ornicar]

I was also doing it, differently, but at the same time

[niklasf]

That's better. I'll not touch lila, then.