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.
The target service name remains unused in its current form, but will be
required in order to verify that an incoming request with an LSAT
attached is authorized to access the service being attempted. We can
derive this from the request's host field, but we choose to extend the
methods with the additional parameter in order to prevent parsing the
host field again to determine which service is being accessed.
