We move the proxy from using its placeholder macaroon creation and
verification to instead use the agreed upon macaroon design. Much of
this is solely a refactor, but some new functionality has also been
introduced as part of integrating the LSAT mint:
1. A request's target service is now verified to ensure its attached
LSAT is authorized.
2. The preimage is now checked against the token's committed payment
hash to ensure it has been paid for.
