In this commit, we add a new struct, the `macaroonPouch`. This struct
bundles all the macaroons we need for each sub-server. Along the way, we
also export the set of default* paths for lnd, and add a new set of
variables that store the default file names of each of the macaroons for
each sub-server.
The `WithMacaroonAuth` method on the `serializedMacaroon` type will
allow us to attach a macaroon for each call rather than attaching it at
the connection level. This slight change will allow us to use multiple
macaroons over a single RPC connection, rather than a single global
macaroon.
