Protocol 2.5 - Borrow cap feature

[The-3D]

One of the features needed for protocol 2.5 is the implementation of a borrow cap functionality. The borrow cap will allow to set a maximum amount (expressed in unit of the underlying asset) that can be borrowed out of a specific reserve. If the cap is reached, it will not be possible to borrow the asset anymore.

Implementation details

• Add a new field borrowCap to the reserve configuration bitmap, https://github.com/aave/protocol-v2/blob/master/contracts/protocol/libraries/configuration/ReserveConfiguration.sol

• In validateBorrow() https://github.com/aave/protocol-v2/blob/master/contracts/protocol/libraries/logic/ValidationLogic.sol#L136 add a check that ensures that the borrowCap is either zero or not crossed by comparing it with the total supply of the corresponding debt token. if the borrow cap is crossed, revert

Gas considerations

See if it's possible to find a way to avoid calling the DebtToken.totalSupply() as it can be gas expensive

[dhadrien]

#91