Added preset payouts along with Tap/FlapTap and conversion to ERC20 dai

[hexonaut]

This is a rather large update to keg to move it towards compliance with the governance's requirements: https://forum.makerdao.com/t/flapperdistributor-a-way-to-distribute-system-surplus-while-minimizing-governance/4591

Notable changes:

• Switched to ERC20 Dai for a couple reasons:
  • Using transferFrom() with pre-approval allows us to pull the Dai in and distribute it with one call which allows us to push Dai into the keg instead of requiring authed contracts. Anyone can pay into the keg now without admin access.
  • The common use case of distributing funds to a treasury or something will require moving the Dai into ERC20 form anyways. We save some integration hassle by assuming this as the common case.
  • Added flights which allows for a pre-authorized distribution of funds which can be specified by supplying an arbitrary string. This allows governance to do stuff like set an "operations" budget which distributes 50% to smart contracts and 50% to risk domain teams.
  • Added Tap contract which is used to connect the surplus buffer with the keg. A tap has a pre-authorized rate at which funds can be drained from the vow. This allows us to set say a weekly budget for a particular flight of 20k Dai / week for example. Anyone can call pump() which distributes the funds.
  • Added FlapTap contract which sits between the vow and the current flapper. This contract acts as a siphon which pulls some fixed % of funds from the flapper. It is designed to forward all calls to the flapper and behave nicely so as not to change the assumptions made by the vow when either kicking off an auction or cageing the system.

[hexonaut]

After discussion with @lucas-manuel, we decided to pull out all the delegation and accounting inside keg as this functionality is already available within the vat. Additionally, I removed the reliance on ERC20 Dai as I originally just forgot that vat has delegation with hope.