rename the distributor depenency to reserve in the shelf contract

[xmxanuel]

• calling the reserve distributor in the shelf contact is an artifact from a previous version
  • https://github.com/centrifuge/tinlake/blob/develop/src/borrower/shelf.sol#L73
• rename the distributor to reserve in the shelf contract
• change also applies to the deployer contract and depend method in the shelf