We extract common macaroon validating code into a method, and add a
method whitelist, for methods that won't need macaroons.
This give us explicit control over which methods don't require
macaroons, to avoid inadvertently adding RPCs that are unauthenticated.
For now this whitelist contains the WalletUnlocker methods, as the
wallet password is required to open the macaroon db.
This adds a new package rpcperms which houses the InterceptorChain
struct. This is a central place where we'll craft interceptors to use
for the GRPC server, which includes macaroon enforcement.
This let us add the interceptor chain to the GRPC server before the
macaroon service is ready, allowing us to avoid tearing down the GRPC
server after the wallet has been unlocked.
