Ability to save score

[finnbear]

Requirements:

• Players cannot exploit to instantly withdraw from a battle
• Saved scores do not give an unfair leaderboard advantage
• Only the player who saved the score should be able to restore it
• Saved scores should only be able to be restored once

[caibear]

Would require database to hold previous score and some secret identifier.
Can't use signed cryptographic key because they could use it multiple times.

[finnbear]

You can now use the c key to drop score (at 50% efficiency) to your friend. They can drop it back to you later (at 50% efficiency).