rpc: set params for the longest time a connection can be idle

In this commit, we set a gRPC param that controls how long a connection
can be idle for. The goal here is to prune the amount of open TCP
connections on an active/popular universe server. According to the docs:

> Idleness duration is defined since the most recent time the number of
outstanding RPCs became zero or the connection establishment.

server.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"sync"
 	"sync/atomic"
+	"time"
 
 	proxy "github.com/grpc-ecosystem/grpc-gateway/v2/runtime"
 	"github.com/lightninglabs/lndclient"
@@ -21,6 +22,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnrpc"
 	"github.com/lightningnetwork/lnd/macaroons"
 	"google.golang.org/grpc"
+	"google.golang.org/grpc/keepalive"
 	"gopkg.in/macaroon-bakery.v2/bakery"
 )
 
@@ -269,6 +271,12 @@ func (s *Server) RunUntilShutdown(mainErrChan <-chan error) error {
 	serverOpts = append(serverOpts, rpcServerOpts...)
 	serverOpts = append(serverOpts, ServerMaxMsgReceiveSize)
 
+	keepAliveParams := keepalive.ServerParameters{
+		MaxConnectionIdle: time.Minute * 2,
+	}
+
+	serverOpts = append(serverOpts, grpc.KeepaliveParams(keepAliveParams))
+
 	grpcServer := grpc.NewServer(serverOpts...)
 	defer grpcServer.Stop()
 

universe/syncer.go

@@ -461,7 +461,6 @@ func (s *SimpleSyncer) SyncUniverse(ctx context.Context, host ServerAddr,
 		return nil, fmt.Errorf("unable to create remote diff "+
 			"engine: %w", err)
 	}
-
 	defer diffEngine.Close()
 
 	// With the engine created, we can now sync the local Universe with the