Change YFI staking rewards to increased YFI

[tracheopteryx]

@Sinsecato

I’ve been thinking about the idea of creating a YFI vault that does exactly this, uses the YFI system rewards to buy more YFI for the vault holders. And then the next step would be to use the YFI-Vault tokens as the voting tokens, rather than YFI itself.

See this thread How to avoid systemic risk from platforms like YAM, while increasing YFI value - #3 by tracheopteryx for additional discussion of exactly this idea.