Set WBTC Collateral Factor to 65%

This change increases the Collateral Factor of WBTC from 0% to 65%.

WBTC currently cannot be used as collateral on Compound, although it can be loaned and borrowed. This would allow users to loan WBTC while using 65% of its value as collateral.

DAI, USDC, and ETH each already have a Collateral Factor of 75%. BTC, and thus WBTC, has greater market liquidity than any of those and comparable market volatility to ETH, and so warrants a similar Collateral Factor.

Because this is increasing a Collateral Factor it cannot trigger any liquidations.

This proposal makes no changes beyond setting the Collateral Factor of WBTC to 65%.

Prior discussion has generally endorsed this idea but mentioned the risk of large amounts of WBTC minting. We appreciate that risk but believe it to be very low. More importantly, this risk already exists on Compound, between USDC and DAI. Given that the incentive structure has resulted in very large amounts of DAI being borrowed and loaned on the platform, we believe that adding more diversity to the set of usable collateral will decrease rather than increase the structural risk to the platform by moving some usage away from DAI and towards WBTC.

We believe this change is justified and appropriate. We also think that it will help to diversify the set of assets available to borrow on Compound.