In order to be able to register the subservers with the root grpc server
before we have all dependencies available, we wrap them in an
GrpcHandler struct. This struct will initially hold an empty reference
to the subservers, which allows us to register with the GRPC server, and
later populate and create the subserver instance.
In this commit, we implement the full RouterServer as specified by the
newly added sub-service as defined in router.proto. This new sub-server
has its own macaroon state (but overlapping permissions which can be
combined with the current admin.macaroon), and gives users a simplified
interface for a gRPC service that is able to simply send payment. Much
of the error reporting atm, is a place holder, and a follow up commit
will put up the infrastructure for a proper set of errors.
