To make sure no macaroons are created anywhere if the stateless
initialization was requested, we keep the requested initialization mode
in the memory of the macaroon service.
To make the permission system even more fine-grained, we want to allow
users to specify exact gRPC URIs in the macaroon permissions instead of
just broad entity/action groups.
For this we add the special entity "uri" which allows an URI specific
permission to be defined as "uri:/lnrpc.Lightning/GetInfo" for example
instead of the more coarse "info:read" which gives access to multiple
URIs.
