In this commit, we modify all sub-servers as well as the main lnd gRPC
service to use a macaroon per call rather than a connection level
macaroon. The old approach would result in us using the same macaroon
for the entire connection, while this new approach allows us to use
multiple macaroons with a single connection.
We move to this new approach as it doesn't require users to delete their
admin macaroon before being able to use Loop. It also preps us for a
future where there is no admin macaroon, and we instead need to use a
set of macaroons to accomplish our tasks.
