multi: make MsgRouter available in the ImplementationCfg

With this commit, we allow the `MsgRouter` to be available in the
`ImplementationCfg`. With this, programs outside of lnd itself are able
to now hook into the message processing flow to direct handle custom
messages, and even normal wire messages.

server.go

@@ -160,6 +160,8 @@ type server struct {
 
 	cfg *Config
 
+	implCfg *ImplementationCfg
+
 	// identityECDH is an ECDH capable wrapper for the private key used
 	// to authenticate any incoming connections.
 	identityECDH keychain.SingleKeyECDH
@@ -486,7 +488,8 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 	chansToRestore walletunlocker.ChannelsToRecover,
 	chanPredicate chanacceptor.ChannelAcceptor,
 	torController *tor.Controller, tlsManager *TLSManager,
-	leaderElector cluster.LeaderElector) (*server, error) {
+	leaderElector cluster.LeaderElector,
+	implCfg *ImplementationCfg) (*server, error) {
 
 	var (
 		err         error
@@ -571,6 +574,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s := &server{
 		cfg:            cfg,
+		implCfg:        implCfg,
 		graphDB:        dbs.GraphDB.ChannelGraph(),
 		chanStateDB:    dbs.ChanStateDB.ChannelStateDB(),
 		addrSource:     dbs.ChanStateDB,
@@ -3986,6 +3990,7 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 		DisallowRouteBlinding:  s.cfg.ProtocolOptions.NoRouteBlinding(),
 		MaxFeeExposure:         thresholdMSats,
 		Quit:                   s.quit,
+		MsgRouter:              s.implCfg.MsgRouter,
 	}
 
 	copy(pCfg.PubKeyBytes[:], peerAddr.IdentityKey.SerializeCompressed())