lnrpc: wrap subservers in GrpcHandler

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.

lnrpc/chainrpc/driver.go

@@ -13,7 +13,7 @@ import (
 // the config that is meant for us in the config dispatcher, then we'll exit
 // with an error.
 func createNewSubServer(configRegistry lnrpc.SubServerConfigDispatcher) (
-	lnrpc.SubServer, lnrpc.MacaroonPerms, error) {
+	*Server, lnrpc.MacaroonPerms, error) {
 
 	// We'll attempt to look up the config that we expect, according to our
 	// subServerName name. If we can't find this, then we'll exit with an
@@ -55,10 +55,8 @@ func createNewSubServer(configRegistry lnrpc.SubServerConfigDispatcher) (
 func init() {
 	subServer := &lnrpc.SubServerDriver{
 		SubServerName: subServerName,
-		New: func(c lnrpc.SubServerConfigDispatcher) (
-			lnrpc.SubServer, lnrpc.MacaroonPerms, error) {
-
-			return createNewSubServer(c)
+		NewGrpcHandler: func() lnrpc.GrpcHandler {
+			return &ServerShell{}
 		},
 	}