Add Private Lending pool (WIP, untested)

Kashi pair with the following changes:

- Charge protocol fee on liquidations
- Charge protocol fee on borrow open
- Allow only one lender, and a whitelist of borrowers
- Collect fees as soon as feasible; do not "reinvest"
- Add option to seize collateral on liquidation.
  Bonus and fees are taken out in the collateral as well
- Add expiration (liquidation-free period)