account balance hub to prevent "donation" attacks

[georgercarder]

Many issues of the code4rena engagement address "donation" attacks.

There may be more ways the v1 is vulnerable to this so it is worthwhile to develop account balance hub pattern that would only acknowledge "legitimate" balances and even sweep donated balances.

The base of the pattern would actually be quite simple:

• "account balance hub" mapping token -> balance
• use this mapping as "source of truth" for account balances for actions in the platform
• update that mapping with every legitimate increase in balance
• swept donations may go to or other account where it makes sense.