lnd: decouple rpcServer from server, wait till chain synced before starting server

This commit overhauls the way that lnd is created with the goal of
ensuring the chain backends are fully synced up before the daemon
itself starts. The rpcServer has been slightly decoupled from the
server itself s.t we can start the rpcServer independently of the
server. This is required as we’ll now wait (unless we’re in simnet
mode) for the chain to fully sync up before we even _start_ any of the
server’s goroutines.

rpcserver.go

@@ -781,6 +781,7 @@ func (r *rpcServer) GetInfo(ctx context.Context,
 	for i, chain := range registeredChains.ActiveChains() {
 		activeChains[i] = chain.String()
 	}
+
 	// TODO(roasbeef): add synced height n stuff
 	return &lnrpc.GetInfoResponse{
 		IdentityPubkey:     hex.EncodeToString(idPub),